system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The universe is expanding, what is going on at the perimeter?
gpt
Think about an inflatable rubber ball, and imagine yourself as a two dimensional creature living on the surface of the ball. If you went in any direction far enough, you would eventually arrive back at your starting point. Now imagine that the ball starts getting inflated more. There is no perimeter to expand, but everything in your "universe" gets further away from each other as the ball expands.

Now imagine that two dimensional surface as a three dimensional space, and everything else is the same.