[idi banashapan]

Originally Posted by daveeb

You're probably right in that it should have matter represented inside it also, a bit difficult with a balloon. It just shows rather crudely that everything is moving away from everything else. That said it's a 4D (apparently) representation of space-time not just space so perhaps the inside is the time vector and the surface represents 3D space. Difficult to represent 4 dimensions using 3.

I think someone mentioned the raisin bread analogy as an alternative, which I actually think is a better one as that does describe for matter on the inside.