system
Given the following post, respond as if you're commenting on Reddit:
human
How is space expanding? Are objects(such as galaxies, stars, etc) just moving to fill the empty space, or is the actual 'space' expanding?
gpt
Space itself is expanding.  Objects themselves are not moving through space (at least, not very much).

The reason we know that space itself is expanding is that, in any direction we look, we see distant galaxies moving away from us.  It's *possible* that Earth is at the center of the expansion, and that everything is, in fact, moving away in all directions - but that goes against most other observations that suggest the universe is isotropic (more or less the same in all directions) and homogeneous (pretty much the same over large volumes).  If everything were moving away, that puts us in a very special spot, which contradicts the isotropy and homogeneity.  Thus, we conclude the expansion is uniform across the universe.

Furthermore, we observe that more distant objects are moving away from us *faster* than closer objects.  This suggests the expansion of space itself.  If space is expanding, then the more space between two objects, the faster they will appear to be moving apart.

A common analogy of this is baking raisin bread.  As the bread expands, all the raisins in the bread will get farther from each other - but they aren't actually moving through the bread at all.  The expansion of the bread moves them apart, but relative distances and locations stay the same.  Further, *every* raisin would look around and say that all other raisins are moving away from it, and that the more distant raisins are moving away faster (because there is more bread between them that is expanding!).