system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the universe is always expanding, that means that there are places that the universe hasn't reached yet. What is there before the universe gets there.

I just can't fathom what's on the other side of the universe, and would love if you guys could help!
gpt
The best metaphor for this is to imagine the universe is 2D, not 3D, so it's basically on a plane, like stars are painted on a sheet of paper. Now imagine it's not a sheet of paper but the rubber surface of a ballon. Now imagine the ballon is inflating. More and more space (the surface) is created, it is expanding, but it's not like it is expanding "over" empty space: the space itself is expanding.

(This metaphor creates some misunderstanding as well, but works well for your question. Here is where it fails: it leads you to think you need some "3D" space outside/inside that 2D "universe"; you don't actually need that to explain our universe).