system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that space is expanding and it's not only the matter that's being spread across after Big Bang ?

Sorry for simple examples but I think it's going to be clearer.

What we think takes place:

* Space is expanding since the big bang, so we're like ants walking on the balloon that's constantly being pumped.

Why isn't that the case:

* The ballon is finite and stationary, it does not expand. What happened was, on some spot on that balloon matter exploded throwing space debris across it's surface. So in this case space is not expanding, it's the matter that's just being propelled from initial bang.

Both ideas seem to fit to our observations (universe getting colder, galaxies running away from us). But second does not require the space itself to expand. Why can't it be a fixed size ?
gpt
But if the second idea were true, we would expect the observed expansion to be anisotropic.  We indeed observe that the expansion seems to be isotropic, and furthermore, the galaxies seem to be moving faster the farther away they are from us.