system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How the Universe is ever expanding.

If it is ever expanding, what is it expanding into?
gpt
Imagine that you take a balloon and blow it up a little.  You then get a permanent marker and draw lots of spots on it. You then keep blowing into the balloon to blow it up some more. Looking at the spots, you notice that each spot has gotten further away from every other spot. The *surface* of the balloon is a bit like a 2-D version of our 3-D universe: the surface of the balloon grows in area, but there isn't a boundary on the surface that's moving outwards. The spots are like galaxies, whereever you sit on the surface of the balloon, the spots seem to be moving away as the balloon is blown up.

In fact, our universe isn't quite like the balloon. The balloon's surface is actually curved and periodic, meaning that you can go round the balloon and get back to where you started from. The universe is in fact flat, so a better way to imagine it is as an infinite sheet of rubber with lots of spots drawn on. As the rubber is stretched, all the spots move away from each other, but the rubber sheet isn't expanding into anything - it's already infinite in size.