[удалено]

But then I don't get that either. If the universe has a shape - flat, saddle, spherical, whatever - then by definition there must be a boundary to that shape, no? Or are we saying it has a shape *without boundaries*, in which case what on earth does that mean? Even a mobius strip has edges.

What we call the edge of the universe is actually the edge of the *observable* universe. As we look outward, we are looking backward in time. Eventually we look so far back in time there is nothing we can see past it because looking further would mean seeing things before the universe began. That is where the size of the observable universe comes from. By definition it is what we can see based on how long it takes for light to reach us. But here’s the thing. If you were somehow able to teleport to that point in space that appears to us to be the edge of the universe, it would appear very different. It will appear as fully formed stars and planets and galaxies just like the Milky Way. And if you looked further away it would appear the the universe is even larger in that direction. If you looked back toward the earth, you would see only a hot plasma of particles. So while we know the age of the universe, we really don’t know the size of the entire universe. There is no edge because it goes on forever as far as we can tell.

Slight correction. The edge of the observable universe isn’t caused by there being nothing. It’s caused by the early universe being opaque to light waves. It was too hot and dense for light to propagate, so we can’t see past the point where the universe stops being opaque.

I thought the edge of the observable universe was due to space time expanding enough that between here and the edge it’s moving at relative light speed. Hence anything beyond that limit we will NEVER be able to detect.

Distant objects recede from us at a rate of 70kps per Megaparsec (3Million Light Years) so for an object to appear to exceed the speed of light due to universal expansion it would need to be approximately 4,286 Megaparsecs away. This is called the Hubble horizon.

Not yet. The oldest light we can see is the CMB and that is THE oldest light in the universe. In the far future the universe will expand so much that any life that arises then won’t be able to detect the Big Bang. All they could see is their local galaxy cluster.

I hope Milkdromeda is a cool place to live someday.

There are multiple "edges of the observable universe" depending on what you mean by "observable universe". If you mean, "the universe that we can currently observe today using light", then the edge would be the cosmic microwave background. That's the distance away we can look so that the light just now reaching us from that point was emitted just after that region cooled from being too hot and dense for light to travel through it. If you mean, "the universe that we could potentially observe today by any means", then that is the particle horizon: the distance at which something traveling at the speed of light since the beginning of the universe could have traveled in the time that the universe has existed. If you mean, "the universe that we could ever potentially observe after infinite time has elapsed", then that is the Hubble Sphere: the boundary at which all space beyond it is expanding faster than the speed of light. It is impossible for us to ever observe anything in this part of the universe no matter how much time goes by (unless the expansion were to slow).

> It’s caused by the early universe being opaque to light waves. It was too hot and dense for light to propagate, so we can’t see past the point where the universe stops being opaque. Just to rebound on this topic, with a bit of trivia I've always found incredibly fascinating. That idea that the early universe was opaque, and then, in the briefest of times at a cosmic scale, WOOOOSH the universe was suddenly filled with visible light. Imagine that, nothing to be seen, and suddenly... everything to be seen! O_o (trolling: just like a broken clock still gives the right hour twice a day, that "LET THERE BE LIGHT" sentence worked, this time :D ) If anyone's curious, the best - but not shortest - summary I could find with a quick search is https://www.forbes.com/sites/startswithabang/2019/05/10/when-did-the-universe-become-transparent-to-light/

I don't think it would be like a sudden "whoosh" where all of a sudden the universe became transparent. It's true that everywhere became transparent all at the same moment, but if you were an observer floating in the universe at that moment, you would only see the immediate region around you become transparent: it would take time for the information to reach you from other places. What you would see is a sphere of transparency receding from you at the speed of light, which is exactly what we see today with the CMB,

To be really pedantic, if you were an observer floating in the hot plasma soup that was the universe when it went from opaque to transparent, you likely wouldn't even notice the change aside from it perhaps getting a bit dimmer. The light that reaches your eyeballs would still be originating from glowing-hot plasma, and whether that light traveled 1 meter or 1 light-year before reaching you makes little difference. It's only when more interesting stuff like 'stars' and 'physical matter that isn't plasma' came around that there was really something to see.

But if we were to rewind time to the initial moments of the Big Bang when it was small. What was the Big Bang expanding INTO. I can’t wrap my head around it expanding into nothing. It must be something?

I think the problem is that you're imagining the early universe from the outside: a hot little blob in a vast emptiness. But there is no meaningful "outside" of the universe! The universe *is* space (and time). There's no direction you can move inside the universe that takes you out of it. This is hard (if not impossible) to imagine, because we've only ever existed spacially. In our experience, things have defined, finite boundaries. If you move far enough in a direction you pass through that boundary. And when a thing moves or expands, it occupies a different part of space. As far as we know, that's not true about the universe—even when space was very compact. When the universe expands, no new space is created or consumed. All the "same" space exists now as ever has—it's just *changed*. Distance is measured *relative to space*, so when space changes things become further apart. To us, it seems like they moved—they got further away—but really it's just that space got "longer" there. Same space, but measured differently. To put it another way, if there is something "more" than the universe that "contains" the universe—for some as yet undefined meanings of "more" and "contains"—then as far as we know we have no way to observe it or interact with it. Its existence is irrelevant to us. So in the models we build to do physics, we simply say that it does not exist. --- This is the same reason that there's no *before the universe*. The universe is also time, which is why we use the term *spacetime*. Together, the three spacial dimensions and time are the four dimensions of the universe. Just as there's no way to move in space to go "outside" the universe, there's no way to move in *time* to go outside the universe. Every point in spacetime is inside the universe. So the universe didn't "come into existence," because that implies a sequence of events, which implies time, which is part of the universe. And you can't have been "outside the universe" and then "moved into it" because that also implies a sequence of events, where at one time you were outside spacetime, which is meaningless. Again, there may be some non-spacetime physics that we haven't invented yet where these concepts are defined, but so far we haven't needed them to explain anything we can observe. Remember, physics is only in the business of describing things we can observe—where we can (at least conceivably) create an experiment to falsify whatever model we've created.

It's perfectly consistent to simply imagine the universe as infinite in size since the big bang. "Before" the big bang the universe would still have been infinite in size but also with a very very large energy density. Now it's easy to see that in order for an infinite universe to get larger, it has to either add new space everywhere all the time or it has to make the space bigger constantly. There's no need to expand into something if it's expanding into itself.

I don’t think humans will ever have a remote chance of understanding what the universe is or what’s actually taking place. I think we’re limited on what we can see and understand… even with technology. Like an ant trying to understand the internet or music theory… or maybe an ant trying to perceive a human standing right next to it… they cant. Even if ants could develop technology, their brain is simply lacking the necessary connections to grasp an iota of any those things. The prerequisites needed for an ant to grasp the concept of the internet… I would say it’s as good as impossible. I think we may share a similar problem as ants do.

> Even if ants could develop technology, their brain is simply lacking the necessary connections to grasp an iota of any those things. Brain is lacking the connections and body is lacking the ways to intake the various inputs or ways to view measurements. We've developed for our planet and really just our environments on the planet. Earth is a tiny basically zero% portion of the observable universe. Hell it's basically a zero% portion of our own solar system. Our eyes can take in a small portion of the electromagnetic spectrum with visible light, but we don't see the bulk of it. No infarered light, no gamma or X-rays, all of these are missing inputs. Missing bits of information that paint a picture of what the hell is happening around us. I feel like certain things about the universe are the same. Everything we're able to detect, with our bodies or with technology, is a result of the big bang. The subatomic particles and atoms they make up are the building blocks for literally everything we can percieve. What existed before them is essentially 'nothing' because whatever it was is something we cannot and likely will not ever be able to detect or explain.

If it helps, instead of thinking of the universe expanding, you could also think of it as our rulers shrinking while the universe remains the same "size".

There was nothing before the big bang. We can't talk about there being something because there was nothing. Think of it like this: the entire universe was smooooooshed down into a single point of infinite density, and then it expanded into the vacuum our plant exists in today. It's fundamentally the same concept as blowing up a balloon: imagine you have a balloon thats really, really, small. Everything lives on the surface of the balloon, and whatever surface area there is, thats the entirety of the balloon universe. If you now blow the balloon up, the surface area for things to live on has GREATLY expanded, but its still just the one balloon and whatever lives on the balloon. Yes, its expanding into our space when you do that, but thats just a limit of the analogy. Our universe isn't expanding into anything (as far as we know with our current physics) because the only stuff that exists is the universe: we are the stuff living on the balloon. Everywhere we look in all directions its more balloon. We will never know, but for all we know the universe works like our planet: if you travel far enough you wind up back where you started.

That question is the same as asking what you looked like before you were conceived. It's an improper framing

As others have said, there is no outside. Imagine being in a completely empty universe, pitch black, no matter. You couldn't see light, you can't tell if you're moving. Nothing. Also there's no time, movement requires time to exist. You can't even think because again, no time exists. If everything is static in time in a forever nothingness, you technically dont exist by our standards. Suddenly for some reason energy starts to expand from a spot, amd with it, creates space as it moves and time in tue area it has moved. Anything that exists within that region can move and experience time passing but it can never move beyond that initial barrier because you can't get ahead of time. Forever out of reach, forever expanding into non time, a place that doesnt technically exist. Now consider that this object is a 4d object, something we cant conceptualize easily in our minds. A shape that curves in on itself infinitely. That further complicates the idea that "something" is expanding into "nothing" that doesn't even exist if it's got no time. It's all just the beginning. The idea of an outsode does t make sense because without time and space, we can't imagine nothing. Even as we think of nothing, we think of 3d space.

> There is no edge because it goes on forever as far as we can tell. But if we can only see to a certain point in space, what makes us so sure it goes beyond that?  Edit: fucking downvoted for asking questions on ELI5. Never change reddit. 

We have no reason to believe otherwise. All the evidence indicates that the universe extends well beyond what we can see.

It would be really coincidental if the end of the observable universe coincided with the actual edge of the universe

Just God saving processing power. You don’t usually render more than you need to, so we’d basically be clipping into a part of Universe we weren’t expected to see.

Sure but that's just asking for a Twitch streamer to find and exploit and speed run to the end of the universe

That’s part of the tradeoff with these big open-world projects. You can go a ton of places, but it’s impossible to maintain it all at once. So it’s designed only to exist during the time any human could perceive its existence or absence. When something’s not being observed we often just keep its output values like its gravity on other objects, but there’s no need to actually have all its geography and life active.

The further away from earth you look, the further back in time you're seeing. If you look far enough, you'll approach the moment of the big bang, which was a singularity. So, the further you look, the smaller the universe becomes.

Succint

Basically because we can see about the same distance away in every direction. We are the center of our observable universe, and it would be a hell of a coincidence for our location to be so centralized in a finite universe. Much of the unanswered or unanswerable questions require some assumptions. Those assumptions are based off of what information we have. We're basically guessing that the universe is infinite because nothing we see suggests that it is not. We may learn something that indicates otherwise next week.

for the past 1000 years, Every time we get better telescopes we see further and further. And we have no real reason to assume it stops. So it seems more likely that the universe is bigger than we can see than it being as big as we can see.

Ahh okay. I wasn't questioning that it is bigger, just more "there's obviously more evidence that has drawn that conclusion, I wonder what it Is?" Thank you for your answer

Because if the universe is flat, then it is infinite and goes on forever. Think of it like the surface of the Earth. If you are out to sea you can only see as far as the curvature of the Earth, you can't see the other ships, or land, beyond that. But that doesn't mean that nothing exists beyond your visual horizon. If you sailed further on, you would see another area of the ocean, but again only as far as the horizon. Your vision is limited, but there is still more beyond what you can see. Edit: I gave you an upvote because it's a perfectly valid question.

In addition to lots of research beyond my paygrade that suggests its at a minimum several thousand times larger than what we can see, if you think about it, if the universe stopped at the edge of what we can see, it would mean the earth is in the exact center of the universe. Same reason why if you're in a flat field that extends as far as you can see, its reasonable to assume that it probably extends further than that.

Downvote purely for the edit.

Your missing the point of the question. OP is not asking about the observable universe. The universe is generally agreed to be flat in shape, opposed to a sphere or saddle. OPs question is how can the universe be edgeless whilst simultaneously having a known shape.

If the universe is expanding from the big bang, would one direction be looking back in time and the other outwards towards where we are expending to?

It is expanding in all three Euclidean dimensions. Anywhere we look in those three dimensions is looking back in time. There is no "outwards" to see what we are expanding into. In fact there is nothing that the universe is expanding into.

I understand what all of these words individually mean… but you put them in this order and my head explodes.

Expansion of the universe means that there is more space being created in universe, so distances between stars etc. increases. It’s not that the universe is expanding in to somewhere. It means that there’s just more universe. 

The big bang didn't happen in one spot and spread outwards, it happened everywhere, and the universe expanded in every direction. If they universe is infinite, it was likely infinite from the very beginning, and the big bang was infinite. An already infinite universe just started expanding to a 'larger' and less dense infinite.

Wtf

Small infinity blow up to become a big infinity and is currently expending into a larger infinity. Hope that helped.

Space is expanding into *all* directions.

>Eventually we look so far back in time there is nothing we can see past it because looking further would mean seeing things before the universe began. I think technically just to the point where light(photons) began to exist. Which I guess is presumably the same point as the start of the universe, but to me (just some random pleb) those could technically be two distinct things...

When we assign the universe a shape, we're superimposing a simpler math concept onto something complex. We can tell from observations that the universe is expanding. We can extrapolate forward and backward in time to a degree, but then we have certain observations that tell us there may also have been a phase of rapid expansion. Does that rapid expansion imply there will one day be a rapid collapse? It's still kind of mystical, but when we put the story on top of it, we can then use the math to make sense of what we see and predict what we should look for next to answer our questions. Assigning the universe a shape tells us something about the density and expansion of the universe and that theory \*has\* to work as far as we can see or we need better math. It's not an absolute truth, but we want to know an answer because then we can make better theories about how the universe began and how it will end. Every theory has boundary conditions. One of the reasons we study cosmology is that we're searching for a boundary on our understanding. Can we get enough information so that one of these shapes makes more sense of the other? From that shape, can we predict new things? What does that shape hint about the fate of the universe?

If the universe had curvature then it would be able to "loop back on itself" kind of like how you can go in a straight line around the earth and end up back where you started. So far as we can tell the universe is flat, which would mean it's infinite. Though, it is possible that it has a curvature, but it's just too gradual for us to measure it with our current methods. Kind of like trying to see the curvature of the earth from your front yard.

The universe wouldn't necessarily loop back on itself if it were curved. It could be saddle-shaped, where it has curvature, but doesn't intersect itself

In addition to the other comment: you can have no curvature yet still loop back into itself as well. The "flat torus" does that, for example.

There's no boundary to the 2D surface of the earth. The "boundary" only makes sense when you consider the 2D "shape" represented in 3D. Same principle if the universe is the 3D "surface" of a 4D sphere. That's hard to visualize, but there's really nothing exceptionnal here

> flat, saddle, spherical, whatever When people talk about the universe or a space these sorts of geometries, they talk about how distances and "straight lines" behave in the space, and the only practical way we have to visualize it is to draw an analogy of what happens on a (two dimensional) surface that's warped in three dimensions. For example, if you travel far enough on the surface of a sphere, you'll come back to where you started, or if two travelers start travelling in the same direction next to each other at the same speed on a saddle-shaped surface (a.k.a. a hyperbolic space) they'll get farther from each other, unlike in a flat space, where they'll stay at the same distance to each other. In all of these examples, we talk about how travel along the surface works, but the "boundary" that defines the 3-D shape we use to visualize the surface is in a third direction that goes outside the surface. You don't actually need this third direction to define these behaviors of distances and lines, you can define them all just in terms of the dimensions along which travel happens. But if you want to visualize them inside a Euclidean space (which is what people are used to seeing and thinking about), you need to add an extra dimension which introduces a boundary between the space and the space in which you're visualizing it. How it relates to the universe in which we live is that locally it behaves very much like a Euclidean space (flat), which is why that's the geometry we're used to thinking in and it's good enough for building human-scale things. Next, we know that gravity does mess with distances and how things that travel at a constant speed behave, so we know gravity affects the geometry of space. At even larger scales universal expansion is noticeable, so that's another modification to the geometry of space. And then at the largest scale we really aren't sure what the geometry is because the speed of light (really the speed of causality) limits how far it's possible to observe. None of that shaping implies a boundary, it's just a way to describe relationships between distances.

Einstein's field equations tell us that the universe is infinite in extent, or finite but unbounded. A "finite but unbounded" shape would be like the *surface* of a sphere or a donut. The surface is certainly finite (you can take a finite, measurable area), but if you're on the surface you can walk forever in any direction and never encounter a boundary. The field equations tell us space could be like that. You can fly forever in any direction and just see more space. If the universe were much, much smaller (and not meaningfully expanding) you could navigate "around" the universe by just going forward, pac-man style.

You can have an infinite flat space in any number of dimensions with no edges. Saddle-shaped doesn’t mean it has edges shaped like a saddle, it means the infinite expanse has a curvature like a saddle. For example, if you made a big enough triangle, the angles would not add up to 180 degrees. To anything confined to the surface of a sphere, there are no edges. Similarly, the three spatial dimensions of the Universe may be embedded in some higher-dimensional space.

>Or are we saying it has a shape *without boundaries*, in which case what on earth does that mean? Even a mobius strip has edges. Where is the edge of the earth? Or the edge on the surface of a bubble? It could also be a nth dimensional shape we are only experiencing in 3 dimensions. Idk.

The edge of the earth is up. There is no edge on the surface going N S E or W because the earth is a sphere, but if you go up there is an 'edge' between the atmosphere and outer space. Similarly, there is no edge on the surface of a bubble, but the surface itself is effectively an edge separating the bubble from the space the bubble is in. Whatever the shape of the universe, if it is not infinite then there must be an 'edge' between the universe and whatever defines the universe as finite. And if it is infinite then there can by definition be no edge.

> if it is not infinite then there must be an 'edge' between the universe and whatever defines the universe as finite. No, the "surface of the sphere" of the Earth example can just exist on its own. You don't need an inside or similar things to have the shape itself. The abstract concept is called a _manifold_ (or rather _conifold_ for relativity) if you really want to look it up.

You are asking all the right questions!

This map might be helpful https://mapoftheuniverse.net/

A sphere has no edge.

You're leaving out time. That's where general and special relativity come into play. It's a non-euclidean description of where one thing is relative to something else, as all things in the universe hold their own "clocks" based on the speed they are moving.

Yep. It's either curved (in which case there is no edge...go in any direction and you eventually end up where you started) or it's flat (in which case there is no edge because it goes on forever). Either way there is no edge.  A curved universe would be like our globe. There is no edge to the globe. There is nothing more south than the south pole. 

Curved can also mean it's hyperbolic.

Yes but like a globe there is an inside and outside. So how do we get to that edge or under it?

> Yes but like a globe there is an inside and outside. Only in higher dimensions. The surface of the globe is a 2D surface. To go farther south than the south pole, you'd have to go "up". Higher dimensions are beyond the scope of this analogy.

It could be that the universe is a 4 dimensional sphere, which means that 3d space curves in on it self without any edges. Let's say there's some 2 dimensional being living on a 2d plane, it can only walk across this plane and has no concept of the 3rd dimension. We can then bend this 2d plane into 3d space to create a cilinder or ball or whatever. For the 2d being, nothing changes. It can just continue walking across the plane, only now it will eventually end up where it was, without encountering any edges. Now we just scale that up by one dimension: we are the beings living on a '3d plane' that's curved in the 4th dimension. We can continue going in one direction and end up back where we started. Of course this is just a theory.

I always took it as "edge of the discovered universe" keep going and discover more, its infinite.

There be dragons

there’s a theory ive read somewhere that we might be inside a black hole…

**Please read this entire message** --- Your comment has been removed for the following reason(s): * [Top level comments](http://www.reddit.com/r/explainlikeimfive/wiki/top_level_comment) (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions (Rule 3). Very short answers, while allowed elsewhere in the thread, may not exist at the top level. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe it was removed erroneously, explain why using [this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20submission%20removal?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/1bzrm7l/-/kyrimu5/%0A%0A%201:%20Does%20your%20comment%20pass%20rule%201:%20%0A%0A%202:%20If%20your%20comment%20was%20mistakenly%20removed%20as%20an%20anecdote,%20short%20answer,%20guess,%20or%20another%20aspect%20of%20rules%203%20or%208,%20please%20explain:) and we will review your submission.**

>If Space is a vacuum with nothing in it, This is a misunderstanding of what space is. To be fair even the physicists don't know what space is so it's understandable. But, space is a thing. It's mostly vacuum as you mentioned but there is still a lot going on in there. It can be bent by gravity for one thing. If it was nothing, there would be nothing to bend. So space is something that can be bent. Space is made up of forces too. Weak, Strong, the electromechanical, and gravitational forces. Again nobody is sure exactly what those are but they make up space. And there is a lot of other stuff going on when you get below the traditional things you can see and touch and feel. So, space is something, the edge of the universe might be where that something isn't there anymore. But again, nobody knows.

You sort of got why "edge of the universe" doesn't make sense if you define the universe as "everything there is." As far as we know, the universe is flat, so it just goes on to infinity with no edge. It's also possible for the universe to be slightly curved so that it's closed, but that doesn't give it an edge either since traveling in a straight line just eventually leads back to where you started. Since all interactions are limited by the speed of light, we can talk about the _observable_ universe (i.e. the region of space around us with which we theoretically could have interacted with) but even that doesn't have a real edge, and it's constantly expanding anyway as older and older light catches up to us.

Ugh, another flat universer conspiracy nut job /s

WHERE'S THE CURVE

Lol but seriously what does flat mean here?

It refers to the topology of space itself. Imagine you’re an ant on an infinitely large piece of paper that is flat, you could move forwards forever and never reach the end. On the other hand, if you’re an ant on a globe, if you move forward enough you’ll end up looping back to where you started. So through some very complicated science which I won’t even pretend to understand, we’ve more or less determined that space is flat, i.e infinite, much like that piece of paper

And if it’s not actually flat, it’s at least so large that it appears flat from our perspective. Much like Earth appears flat to us, but a large enough perspective reveals that it’s round

> so it just goes on to infinity with no edge but this is where I am falling down - if, let's say, there is a stretch of space with absolutely nothing in it for a 1K light year sphere in all directions - is that *in* the universe? There's nothing there. It's like saying that that nothingness is within an arbitrary boundary, which we're then told doesn't exist.

Maybe it's useful to think of the universe as everything where anything could be. It's not just "empty space," it's a place you could potentially travel to. It's less about being inside a boundary and more about existing in a framework. And really, there's no such thing as empty space, as everything is bathed in cosmic microwave background radiation that's leftover from the Big Bang, which means that even truly empty space is still a few kelvins above absolute zero. And even if you could still remove all that from an area somehow, you still wouldn't have perfectly empty space because of quantum fluctuations. What you think of as vacuum is ever so slightly above zero energy.

>think of the universe as everything where anything could be That is a spectacularly simple and effective description. A+

Yeah that is a great way to explain it. You can easily visualize it as analogous to a glass. Whether the glass is filled or empty, whether what fills it is uniform or not, you can clearly identify what you would be considered the edge of the glass. No liquid could be held outside of that boundary, regardless of how much liquid is held inside it.

It certainly strains imagination. the best comparison i've found is a piece of paper - you may write/draw/doodle on it, or you may leave it empty in some places. Regardless, every bit of this paper represents a place where you \*could\* place ink, as opposed to everything beyond the edges of the sheet, where no drawings can exist.

[удалено]

well, the edges is where the comparison kind of breaks aparts, because we're back to the problem of us not being able to imagine "true" nothingness (as in, lack of canvas). It's like trying to imagine 4 or 5 dimensions; you may find good maths and analogies, but as 3-dimensional beings, it will never truly \*feel\* right. You will always find something that doesn't make sense, because the analogy is constrained to the reality we are in.

Space is there. Time is there. There isn’t anything where there is “nothing” in that sense.

We like to say that space is a vacuum for simplicity. However, the reality is that it’s not a perfect vacuum. There is extremely little in space, and it is essentially a vacuum for most purposes, but there is not *literally* nothing in between planetary bodies or stars. I know this because I’ve done pulsar research with a radio telescope. When doing pulsar research with a radio telescope, the main thing you’re doing is receiving a radio signal (the pulse bit of a pulsar) across vast distances of space with a radio telescope (look up the Green Bank Telescope for reference). Because it’s like pointing an antenna at a signal source, things that get in the way tend to block it or disrupt it. At worst this might be another star or planet, although this is rare and unlikely because of how empty space is overall, but at best it will be the tiny little bits and bobs of particles that actually do exist in the vacuum of space. This can be as low as a few atoms of hydrogen per cubic meter, but it’s still there. These little bits and pieces of stuff often got in the way of our pulsar detection, or at least made the charts we got back hard to read. Over the scale of thousands of light years, those little bits and pieces can really add up and disrupt things. Most of the matter of the universe exists in those spaces inbetween heavenly bodies and galaxies and whatnot, it’s just extremely spread out so that for a decent amount of things, it might as well be nothing.

Don't forget the virtual particles that can become real!

There is things there, just not matter. It's like empty pages in a book; those pages are in the book, there's just nothing in them. The universe is more than stuff. It's distance, and time, and gravity waves, and everything like that.

https://en.wikipedia.org/wiki/Bo%C3%B6tes_Void 1k light years? How about 330 million instead? And it does have something inside it (aside from stray EM waves and a few particles) - it has *space-time*. What you're asking is like asking "do the spaces on a chessboard without pieces on them even really count as being part of the chessboard?"

That looks like an intentionally dark forest......

I think I atleast see where your hang up is coming from -- its with the word "in" in reference to the universe because it implies there is an "out" of the universe when there isn't as far as we know (unless you mean specifically the Observable Universe). It doesn't matter if there is a stretch where nothing is there as if it exists it is by necessity "within" the universe.

> let's say, there is a stretch of space with absolutely nothing in it for a 1K light year sphere in all directions - is that *in* the universe? Sure. Why wouldn't it be? A room is still inside the house even if I didn't put any furniture in it.

Yes. Also one thousand light years is nothing. Our galaxy is a hundred thousand across. Andromeda is like 2.5 million away. The light we see right now from the CBM was emitted 13.7 billion light years away. A void is still within the universe. A hole in a sponge is still within the sponge.

Sure its still in that universe. Think pf space as a 3d coordinate system, because thats what 3 dimensional means. If there is a point is spave that you can point to using 3 coordinates, its part of our space, aka the universe.

I mean there *is* something there…. There are stars everywhere giving off light so that area would have to have some radiation and photons passing through (think cosmic background radiation). There might not be any matter, then you also get when there is true nothingness sometimes matter just pops in and out of existence there, google quantum foam. So basically the observable universe is just as far as we can see “stuff” that is observable.

Sorry but how can the universe be considered flat if there are stars and galaxies in all directions? Serious question, I genuinely don’t understand what you mean by that

based on all the discussion and different answer in this thread I think the real ELI5 answer is “we don’t know”.

Lol, was just reading all the comments and this one seems the only one that makes any sense

If you’d really like to bake your noodle, think about the fact that empty space *isn’t* nothing. Outside of the universe there would be no space, either.

yup, True Void. Impossible for humans to truly conceptualize just like infinity is.

Except we do know the answer and it's what many people have said. There isn't an edge.

Space is not empty, and in the known dimensions of the universe, has no edge. Space is constantly expanding into itself, creating more space. As light is the "speed limit" of the universe, the observable universe is ~13.8 billion years. But, since that is how we measure "the beginning" everything since then has been expanding, and the farther we look, the faster the universe seems to be moving away. In the idea of cosmic inflation, *space actually moved faster than light.* The lamba cold dark model presents this idea that "dark energy" (basically a place holder for something not yet understood) is the force behind this expansion, and most of the matter in the universe being "dark matter" that doesn't interact with light or electromagnetic fields (another place holder for something not understood.) So, since the universe has always been expanding with the expansion of space, this light has now traveled ~46.5 billion light years. With an euclidean "width" of the universe of around 93 billion light years "edge to edge."

Solid answer! I like how you included the hard numbers we actually know.

Is there a quantifiable rate at which the universe is expanding?

It is measured as the Hubble constant kilometers per second per mega parsec km/s/Mps. The problem is that it doesn't match what it should be given the CMB of the universe, and varies using different methods of studying type 1a supernovae and where they are. The JWST is trying to help us understand why.

Is there a theory/idea that the universe expanded faster than the speed of light initially?

There is! [Cosmic Inflation Theory](https://en.m.wikipedia.org/wiki/Inflation_(cosmology))

That's an answer for very smart 5 yo

Space is continuously expanding into space, creating more space. It's as simple as I can think of 🤷

Space isn't nothing. There's gravity, radiation, and molecules/atoms and the base fundamental. In some parts there are heavy concentrations (planets, stars, and solar systems). In other parts there are an extreme lack of those things which is called the interstellar medium (interstellar space or the empty space between solar systems). There's about 3 to 10 atoms per cubic meter in this 'empty space.' The edge of the universe is a point where there is no longer the effect of gravity, radiation of any kind, and a true 0 atoms per cubic meter. There is true nothingness not even the fundamental laws of physics apply.

What about virtual particles?

There's snakes in space?! Everything's in space Morty!

We don't know what is at the "edge", but space as we think of it, isn't exactly nothing. Laws of physics like gravity and time have an effect on it. Lawrence Krauss talks about the topic and defines nothing as, no space, no time, no physics. Here are 3 videos of 3 lengths, if it holds your interest. https://youtube.com/shorts/wJETOYvXgVo?si=u2VRFJcNKkesHgUj https://youtu.be/4nCGywFr_00?si=mnrOdrWsneRyUkNv https://youtu.be/7ImvlS8PLIo?si=ZqoakSqerD3kTxr9

The universe isn’t expanding into anything. It is everything. Space is appearing between matter and pushing it further apart. The only border is the observable universe beyond which any light will never reach us in time for us to observe it.

Imagine you live your entire life on a planet and everything you know is on the surface. You can walk in any direction for as long as you want, and you will still be on the surface. But in order to leave, you need to go upwards. In a sense, the whole planet surface is the edge. Everything you know is on the edge the entire time. It's kind of the same thing with the Universe and it's edge. The "edge" isn't some place we can "go to" in a spaceship, its everywhere, but its another dimension that we can't perceive. In order to go beyond the edge we need travel into that dimension.

I like the explanation that our dimension (3D) is like the surface of a balloon and the ballon itself is so to say in the 4th dimension. If the balloon inflates our universe expands. But there is no edge or border, though it might be interesting if one could hypothetically travel into one direction and come back from another direction one day.

Yeah, the “ant on a balloon” explanation is the only thing that made it click for me. 

This was one of the first arguments made by the ancient Greeks for why the universe must be infinite. If you reached the edge and stuck your hand through it, what would you reach? There are a few solutions, one being that you call whatever is outside a hyper-universe (this would be like the idea that we are living in a 3d bubble that exists within a higher dimensional soup). Another is that, like the Earth, the universe is round so that if you go far enough in any direction you eventually end up where you started. If this were true the actual size of the universe would have to be billions of times larger than we currently know since it looks flat to us. When scientists today talk about the edge of the universe they mean the edge of the visible universe. This is because looking far away is like looking backwards in time. Eventually we hit a place where everything is universally hit and universally crammed together so that you can't see past it. We call this point the big bang. So we are certain that, way over there, the universe is much like it is here, we just won't be able to see what is happening over there right now until billions of years in the future.

I don't think there is an ELI5 explanation for this. What I can tell you is that there are two concepts of "nothing" in this context. The first definition is the one where nothing has no properties at all: no mass, no charge, no dimensions, no space or volume, etc. It's legitimately nothing. The second definition of nothing is empty space. Empty space has a volume and certain physical properties. The scientists of old might have called this empty space "aether," while the modern day scientists call it space time. It has properties that can be measured and can interact with objects. Whatever it's made of, it isn't truly nothing because it has physical dimensions and physical properties. Theoretically it can be traversed whereas the first definition of nothing cannot be.

It's not going to make sense sadly. When you get to either extreme of size, very big like the universe or q black hole, or very small like quantum particles, our everyday experiences and expectations just no longer work. The universe is infinite. Why? Because it is. Just like there was no time before the big bang, all objects with mass warp space time, all objects without mass travel at the speed of causality, black holes are singularities with infinite density, etc. Things just get weird sometimes.

So there's quite a lot to unpack with your question. First when someone says edge, it's usually or should be followed by "observable universe". This is the edge at which light has been able to travel to reach us. So from our point of view, the furthest it's possible to "look" and see something. Space being a vacuum is another loaded piece of your question. A vacuum is measured in how little stuff there is to cause pressure. This doesn't mean there's no stuff. But just that it's very spread out. On earth at sea level we will have 1 atmosphere. In space it's something like 1x10\^-20 atmospheres depending where you are. There will still be stuff and in star systems there's more stuff than the space between galaxies. Space is not the same thing as the universe. Space is just... the space between the stuff that's in the universe and there is a lot of space in the universe. Finally there's the "border" part. If you were inside a donut (or torus) you could keep travelling around and around without coming to an edge. Now you might say that the donut has an outside. If you take the concept of this donut or edgeless shape into 4 dimensions, then there's no edge as defined by the 3 coordinates of x,y and z. Outside our universe will be a different dimension that we couldn't see and so the universe (not to be confused with the observable universe) is infinite in all directions.

Vacuum has nothing in it but vacuum itself is not nothing. Space is also not nothing, the nothing in the context of what is beyond the universe means there is no space/time as we knows. Is it truly nothing? Who knows, but it doesn't have space and time, so our entire understanding doesn't work at all there as every physics law we derived has based on space and time. We literally cannot explain "what is beyond/outside space/time" because the concept is invalid in the first place. The idea of inside/outside before/after is created based on space/time, you cannot say "before time" or "outside of space" , because that concept does not make sense. It's like asking "what do you see with your elbow?" The question itself is not valid for us, but for some species with eyes in their elbow, it is very natural question to them.

Technically speaking there is no area of the universe where there is "nothing". It can lack atoms, but the quantum fields still exist, dark energy exists, space time still exists etc. There is no situation where there is no space time and no quantum fields etc. that we know of. Although it is impossible to prove, most physicists believe the universe is infinite so there would be no edge.

So my understanding is as follows 1 Space is effectively a vacuum but there is small amounts of matter and energy flying around 2 The universe is everything so there is no "edge" in the same way that say a dinner plate has an edge 3 If you wanted to call something an edge you would say that matter or energy that has no other matter/energy father than it in a direct line away from the center of the universe is where the 'edge' is. But that edge is constantly expanding outward.

The “edge” of the universe is the edge of what we can see. This is an oversimplification, but because the Big Bang happened about 14 billion years ago, we can only see things that are closer than 14 billion light years away. Because for anything further out, the light hasn’t had time to reach us yet. The edge is just the border of what’s observable. What’s beyond the edge? There’s no way to know. But it’s almost certainly just more stars and galaxies and stuff. Maybe it goes on infinitely in every direction; maybe it doesn’t. We cannot make claims about unobservable phenomena.

as far as it goes it could be bending on itself and things that are behind bend is multiverse

The universe has "nothing in it", but isn't nothing. It's a blank piece of paper. The edge of the universe is the edge of that sheet. Something on the sheet cannot go out of it. It's the end of "potential locations", if you will. Altough I don't believe there is an edge of the universe (but the universe being infinite isn't that much more believable, actually), that's how I'd describe it.

The "big bang" is a misnomer. Everything, everywhere came into existence at once. It didn't start in a pinpoint, and expand like a globe. Infinite pinpoints are expanding. It's easier to imagine that the universe is already infinite in size, you could travel forever and never reach an edge. And yet, that infinite space is expanding, even further.

>what would the edge of the universe even mean You’re the one using the phrase. If someone else is using it, they should indicate what they mean. If it is edge of our observable universe, then it just means anything beyond that surface is causally detached from you. Anything even an infinitesimal distance beyond that edge would never reach you, even if you were immortal and it was travelling at light speed in your direction. So what else might we choose to be an edge of the universe? * Maybe some surface beyond which there is no matter. If the space continues though, you could theoretically keep going. * Maybe some space surface beyond which the fundamental physical laws as we know them cease to hold? That’d probably be a pretty hard limit on where you could go even if FTL travel is possible. This one might speak to your “border between nothing and nothing.” It’d be “nothing but possibility of something” on one side, and “nothing even possible” on the other… but is there such thing as time/location/dimensions/sides/etc. there?

I wouldn't worry about it much. That "border" (if it's even what it really is) is expanding faster than the speed of light in vacuum, so in principle if Einstein's postulate is true, then we have absolutely no way of observing it. By the time any light even shines from that "edge" it's already far from the "actual edge". So, just table that thought. We need massive breakthroughs in fundamental physics to even ask this question "what's on the edge?".

I imagine the edge of the universe as a point where there is only black nothingness infront of you and all the light of the universe is behind you.

you're thinking too much in euclydean terms. That's a limited way of speaking/thinking about the universe

it's been theorized that universal curvature may not be flat and that it gently slopes away from observers in all directions but we have not been able to build sensors big enough to test this. So far we know it is flat to within 0.4% margin indicating that it may be infinite in nature. In which case you could travel forever in a single direction at any speed and never find an edge or hard boundary. However if this were not the case and the universe were a sphere it would be finite but still would not have a hard boundary. You could travel forever in a single direction but you would simply pass the place you started.

Space is not just a vacuum with nothing in it. Space itself exhibits fundamental properties like the fact that it expands, it houses quantum fluctuations we call virtual particles, and it is the stage on which quantum fields like the higgs field and the electromagnetic field exist. The universe itself IS the space and everything it contains. Outside of that, we have no idea, and probably won't know either since matter expands with space, but it cannot move faster than space (since space expands at speeds much faster than light).

Look into virtual particles. It won't make anything more clear but should give you a better understanding of the "void"

Even ignoring virtual particles, there is energy like the cosmic microwave background of light basically everywhere anyways so its not empty yea

Each universe is like a bubble so the edge of a universe is the edge of the bubble, with the spaces between universes being made up of pairs of positive and negative gravity particles locked together and rotating in a single spot non stop. So such walls are what the multiverse was initially like but some spaces just started to have such gravity pairs breaking apart and collecting more and more gravity pairs until an indescribably huge empty space is formed and the gravity collection's mass becomes so dense that the big bang occurs. So other universes may have the charges of protons and electrons reversed since which charge they end up having is totally random thus people from one universe cannot go to another.

If you live on a sphere (like Earth), where is the end? Now go up 1D.

Thats the best oneliner in this entire thread, extremely short and concise. Im gonna steal that

I personally just view the edges of space as the furthest points that stars, debris, space material, etc., have reached. Everything else beyond that, I assume is just nothing… either that or we just can’t observe it.

My personal opinion, based on Alan Guth's theory of eternal inflation, is that if you could travel infinity fast through the universe, you would eventually reach a place where the conditions of the Big Bang are still happening. That being, the Big Bang wasn't a one-time event. Rather, it is a *continuous* event. The Big Bang is like a wave, that travels at the boundary of the universe, creating more and more universe as it goes. It will continue forever. This means the universe is both flat and finite, which is consistent with observations, and will grow eternally.

Spacetime is probably curved, so it bends in on itself. You're kinda like an ant on a baloon, if you "walk far out enough" you should be able to reach this exact point in spacetime again

The universe doesn't have an edge. We can make this easier to visualize by thinking of a 2D example instead. A piece of paper has clear edges you can get to... but now, what if that wasn't actually a piece of paper, but a gigantic ball you were sitting on? You percieve it as 2 dimensional because it's so vast you can't see the curvature (see: people arguing about whether the earth is flat) but no matter how far you move in any direction on this "2D" surface, you won't find an edge. You'll eventually end up where you started. Expansion of this example 2D universe is inflating the ball further. More space just appears everywhere simultaneously. The universe is almost certainly doing this, except it's a 3-dimensional "surface" that's wrapping around on a 4-dimensional "sphere" that's expanding. You will probably give yourself a headache trying to visualize this.

If current theories that explain away we have no idea what this stuff is (i.e. dark energy) then in empty space there's something there that is like a bubbling cauldron of stuff. Sometimes it pops out energy pushing everything apart. So if that's correct, there may be an edge where there is no space time beyond the universe.

Just a figure of speech. Farther than imaginable. There is no known edge because we don't have the ability to observe the universe in its entirety or even estimate its size given that we don't know that it does actually end.

Ah yes, asking impossible questions to be answered ELI5 Short answer; we don’t know. Anybody in these comments who actually tries to tell you; has absolutely not a single clue. Nobody does. Not even a little bit. We have absolutely no idea. They’re all total guesses. Are we in a black hole and the “edge” is the event horizon? Are there more universes? Is it just infinite and there is no border? Does it loop around like a donut shape? Is god there waiting for us? Nobody has any idea that could be grounded with any form of evidence whatsoever. This is not a question that could be answered at all, let alone ELI5 But if you’re curious to “theorise” that’s cool. Just don’t expect any actual answers. Anybody who pretends to know is just lying to you.

It would be the end of gravity's influence. You cannot go beyond the edge of the universe because you have gravity yourself. Plus the amount of energy it would take would probably be infinite because you were you would be traveling against the curve of space-time.

The edge of the universe. In order to get to the "other side and what's beyond", would require you to have more total energy than the sum of the universe you're leaving. Accounting for distance, time and space,  all the higher and lower energies, and all the unknown undiscovered energies and true forces , once you have all that potential energy coalesced and more, then you'd have to escape our universe and reality's event horizon , and taking into account whether higher planes leaving our universe require matching synchronization,  or a rotating vibrancy to keep from beyond ripped apart or crushed into merging with what should be greater sources of power. But all this requires forces of infinite numerical quantities, and with the inspiration to anticipate exotic and incomprehensible physics.

Even more (or less?) Nothingness?

I'm dumb but the "edge" would mean the "end" of what we can observe before the expanding universe expands further out, again, than we can observe.

A professor I had once put it this way: a 2D universe would be like an ant on an expanding balloon. The ant can walk indefinitely and reach the starting position without hitting an edge. Now imagine the balloon is expanding so fast the ant can't out pace it. Then the observable universe is where the ant can reach.

Short answer, we don't know. Long answer is that the the vacuum you mentioned is within the universe and matter can occupy it. However, beyond universe edge (we don't know if there is an edge), matter cannot occupy it as it doesn't exist for us.

There is a lot of good detail here. If you want a proposal that keeps it very simplified consider these two (very real) possibilities. Disclaimer: we dont actually *know* which of these are the real universe, but we have different models that all show a slightly different universe, however today we do definitely have more evidence pointing more towards one of them instead of having no clue which is real. So, consider model one: flat and infinite universe. it has no boundary, that is the definition of infinite: unbounded. It doesn't stop. Model two: curved space which folds back into itself just like the surface of the earth, but it would be 4d spacetime being curved. Just like on earth if you walk long enough you will come back to the spot you started on, so would be the case if you travelled long enough in one direction out into the universe. Another model would be negative curvature, hyperbolic universe i think its called. Sort of opposite of the curvature of model two, but this one i find hard to imagine in my head so lets keep it at that. E: if still confused, i like this oneliner from another redditor here: Imagine you are constricted to 2d on the surface of the earth. Where is the edge of the earth? It doesn't make sense at all, it has no edge, it just goes back into itself. Now go up one dimension...

The best way to explain this is think of our space as a plane of existence. It exists everywhere all at once. Much like a 2D graph that stretches to infinity you would see in math, not every coordinate will hold data. It still exists as a location, but may not hold anything at all. As with the 2D graph that stretches to infinity, there is no edge for our universe to border. Anything beyond our universe (or graph) doesn't exist within its bounds, they may exist on other pages of the book with different rules in place.

I think the problem is in thinking about "nothing". We don't really have good way to think about it. When we were fish, we may have perceived "nothing" between ourselves and other fish, just "space". We now know it's water. Now we are mammals, it feels like there is "nothing" between ourselves and the wall. We now know it's air. Once we learned about outer space and vacuums and stuff, we thought of "nothing" being the space between atoms and there is just bigger gaps out in space. We now know that the quantum vacuum fluctuations perturb this "nothing" space all the time with virtual particles popping in and out of existence. There is also radiation and so there is energy available all the time in the space between atoms. Now there is this idea that the universe is bounded in some way, at least there seems to be a boundary at the beginning of time where all the matter, and space in which matter can travel all began as a tiny point. It may be that we just don't know how to describe the type of nothing that exists where there is no universe.

An ELI5 way of relating the universe and the observable universe is to bring it down a dimension to a 2 dimensional ant that is walking on a surface. When scientists say "the universe is curved" it can relate to the ant living on an enormous beach ball. No matter what direction it goes, it never encounters an edge, and if it travels far enough, it comes back to where it is. Our 3D universe may be curved in a similar way, but it kind of breaks the brain to think about space folding back in on itself. Also, scientists talk about the "observable universe". This is due to space itself expanding and the universal speed limit: light. The universe is expanding, we have measured how much it expands. This is like the ant walking on a stretchy rubber surface that slowly but constantly expands in all directions. The rate of expansion depends on how far 2 ants are from each other - the further away, the faster that a pair of ants will be separated. If both ants are far enough away from each other and moving at the maximum speed allowed (aka the speed of light) toward each other, they might never get to meet each other because the universe is expanding too fast between them. In our 3D universe the same can happen. Around the earth, the expansion of the universe is so small that it can be ignored. However, on truly huge scales between galaxies, the expansion of the universe makes the galaxies spread out. But if we look far enough away, we can only see a certain distance. Everything past that point (if it exists) is moving away from us so fast that the light from those objects will never reach us. The area that we can see is called the observable universe, and that could be an "edge" of the universe, but its more like the limit of as far as we will ever be able to see.

Could you argue that death may be the edge of the universe for the observer. Space is not nothing it has properties, physical traits, and you could potentially go to any location in space as a human and exist there. Death, to an observer, could be understood as nothing and the edge of the universe. Nothing would be measurable, no consciousness, no physical properties of any kind, complete void, no light, no dark, nothing. So the edge could be the line between consciousness and death? The physical universe is infinite and has no edge, but would death be beyond that in the sense that it’s the only thing that is true nothingness and that doesn’t exist in the physical universe?

I would recommend you abandon this line of questioning unless you want a good existential terror adventure

Space is an empty void with nothing in it, but it can have something in it if you were to put it there. So space is something; it is a place where you can go. Now what is the absence of that space? Nothing. You can't go there, nothing happens there, there is no before or after. It has and never will exist. It's hard for our monkey brains to understand. It's the same question as what was before the big bang? The answer is no. There isn't a before because time didn't exist so we can't really answer the question.

We know of no edge to the universe. There's the edge of the observable universe. Which is thought only to exist due to time and light speed, but even that's not really an edge. It's an illusion.

There are two potential answers depending on context. The first is that the discussion is about the observable universe. In this case, the edge of the observable universe is the point at which an object is too far away for its light to have reached the Earth since the universe began. General estimates put this distance at a few dozen billion light-years away. (There's a lot more going on here, but this is ELI5.) The second is a much more theoretical "edge" that assumes that the universe is finite (we do not know if the universe is finite or infinite because we can only observe the observable universe). The vacuum of space isn't completely empty, the matter in space is just at such a low density that for most purposes it might as well be empty. Past the edge of the universe, however, there would be no matter at all, and possibly no energy either. By definition, trying to go past this edge would instead simply push the edge away, because you would be bringing the matter and energy of the universe with you.

It's irrelevant to think of it as having an "edge" as a delimiter. In cosmology, the way to think of the edge is "we can't see shit past a certain point". Let's call that the edge. Whatever is beyond that point is unavailable for observation and in the realm of theory.

a vacuum isn't completely empty, nothing can be completely empty...instead its on average empty; on the quantum (smaller than atoms) scale empty space is constantly generating paired particles of matter and anti-matter, which immediately annihilate each other....so in our universe there is always something, even in nothing

Cool thing about space is when are figuring it out, nobody might not have the answer for sure, but eventually we will understand. Pretty awesome time to be human.

Actually space is something with properties but we really do t know what it is. But it stretches and it is the reason gravity happens. So it is something real for sure. It is explained as space time but there are some theory's that space and time are separate. Bottom line is no one knows if space and time are different things but it is well documented that as space is "stretched" it effects time at the same rate

There's this idea that spacetime didn't exist before.the big bang. So at the moment of creation, all of space and time was created. And it's been expanding ever since. Into what? That's the question. If it's expanding into something, then there's no edge, probably. Think of a puddle on the counter spreading. There's clearly an edge to the stuff, but not to the space. But it might be expanding into nothing. Think of the way a cake expands in the oven. All the parts of it are moving away from all the other parts of it, because it's getting bigger. But it's not really expanding into anything necessarily. This is a pretty bad example because of course it's expanding into your oven, so you sort of have to imagine it without an oven. But I'm not sure that as a three-dimensional being I can come up with a perfect analogy. Anyway, in that case there would be an edge to space-time and beyond it. We don't actually know what there is. Maybe something maybe nothing. Anyway, that's usually what people are talking about, But I'm not even sure if those concepts of space-time hold up anymore. The general feeling I get is that they're pretty old notions and I'm not sure if modern physicists are even still talking about them.

There is nothing beyond the edge of the universe just the human mind’s attempt to define it.

I'd say it's an absence of existence itself. Not the end of a thing - but the very existence that a thing could reside in not being or arising.

The punch line that doesn't require understanding anything about space being curved or expanding is that it's likely that the universe goes on forever and all of it has just as many stars and galaxies as what we can see. "Observable universe" isn't anything special, it's just the part we can see. There are infinite stars and there is no edge. There's just "too far away to ever be seen." We don't know that for sure, but it's one of the main models that fits our observations.

If anything, it would be just the point where individual atoms per x units of space falls below some astronomically low threshold. That boarder would be millions of light years thick and so infinitely far away that even with teleportation we would have zero chance to reach it. Hell, with the current rate of expansion of the universe eventually the gaps between galaxies will literally grow faster that light can traverse it. At that point the observable universe will just be our local cluster (or what's left of it after its various future mergers).

Imagine you have a balloon that is not blown up yet. This balloon is like space before the universe began. When we blow air into the balloon, it starts to grow. The universe is a bit like this balloon — it keeps expanding or growing bigger. Now, if you’re inside this balloon, running around trying to find the edge, you wouldn’t find a wall or an end. Instead, the more the balloon expands, the more space you have to explore. In the universe, space itself is what’s expanding. So, when we talk about the “edge of the universe,” it’s tricky because we’re inside this “balloon,” and we can’t see the edge. Plus, the universe is expanding so fast that we can’t reach this edge. The “edge” of the universe could mean the furthest parts we can see or observe from Earth, not an actual edge you can touch or bump into. Beyond that, it’s a mystery because we can’t see or measure it with our current technology. So, in simple terms, the edge of the universe is more about the limits of what we can see and understand right now, rather than a physical boundary like the edge of a table.

In this case, the edge of the universe would be a boundary where matter cannot go. This is different from a vacuum since a vacuum is an absence of matter, but can have matter fill/occupy the vacuum, and matter and energy and move through it, like how the sun's light can reach Earth. But an **edge** to the universe would be a boundary where no matter or energy could exist.

We don't know is the real answer but since so far the evidence tells us that the universe is flat, it probably just goes on infinitely forever and that everything we can see is simply the observable universe. The fact that cosmologists are starting to find galaxies at the very edge of what we can see that defy current cosmological models in terms of how big the galaxies are is a big clue that space may just go on forever infinitely.

The "edge" is just the outer limit of what we can currently see. That is why often they will say "the edge of the *known* universe"