The expansion of the Universe is one of the hardest things to explain because of two features that appear to be true but are extremely difficult to conceptualize:
1) it started from everywhere (no origin point)
2) it's happening everywhere in the same way (no center)
This collaboration with @minutephysics was my best effort to explain this, but it still seems to break brains on the regular. https://youtu.be/Cfg11qQwPzQ
One key thing to note is that the Universe may be infinite and expanding anyway. No boundaries, no "size," but still expanding.
What that looks like to us is that really distant galaxies are moving away from us (more distant, moving away faster). But that's just what it would look like from ANYWHERE in a universe that's expanding the same way in all directions, at all points.
Being in a uniformly expanding universe means that everyone looks out into the cosmos and sees it running away from them. Being in an expanding universe where the expansion is ACCELERATING (as it is now) means eventually distant galaxies will get so far away we won't see them.
Another important point: it's really just the empty space in between things like galaxies that is expanding. Galaxies themselves are not. You are not. (Not because of physics, at least.) Gravitationally bound systems are not expanding. But we're getting more space between them.
Q: BUT WHAT IS THE UNIVERSE EXPANDING INTO?
The short answer is: maybe nothing. Maybe the cosmos is infinite. We don’t see any indication there’s an edge. All we really know is that the spaces between things we *can* see is getting larger.
Imagine you’re an ant on a large rubber sheet. You can’t see any edges, but you see other ants around you at various distances. You see that the other ants are getting farther away from you & each other. Every ant would see this; each would infer the sheet is stretching.
Anyway the weird thing is that as far as physics is concerned, we don’t need anything outside stretching the Universe, we don’t need there to be ANY “outside” beyond the Universe, the Universe could be infinitely large, and it could have always been so & is getting bigger anyway.
And all our observations are totally consistent with that picture: an infinitely large, nonetheless expanding universe with no edges, no center, no exterior.
You can follow @AstroKatie.