system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Universe is expanding, but what is it expanding into?
gpt
It doesn't have to be expanding into anything: if the universe is everything that exists, then there might be nothing for it to expand into. Instead, it's just that the space inside the universe is increasing, which isn't actually the same thing at all.

Don't bother trying to get your head around that idea -- modern physics is full of stuff no human can get their heads around without going insane.