@trans_caracal@kolektiva.social It's expanding when seen from the inside, i.e. things are getting farther and farther apart from each other. Whether it's expanding "into" something is a separate question and, as you correctly point out, perhaps not even valid as a question.
@timorl @trans_caracal@kolektiva.social
Imagine that the world consisted of some number of noninteracting particles. If that was placed in an expanding spacetime and the particles were initially stationary wrt. each other, then distances between them would increase over time.
Now, if the particles were interacting with each other so that they had some nonzero "preferred" (lowest potential energy) distance, then in an expanding spacetime that distance would be (slightly) increased -- their distance would keep increasing (due to expansion), and they would continuously be pulled together by that potential. (This does mean that expansion does work, and I believe that is so.) At any rates of expansion we could be considering that increase is so minuscule it's not observable for things like atomic bonds, but technically at an absurdly fast expansion rates you wouldn't be able to have atomic bonds: they would be pulled apart "faster" than they can pull themselves back together and would be pulled far enough to "break the bond".
So, where can we see "normal" levels of expansion? The smallest objects that basically don't interact with each other over their typical distances (I'm being sloppy and vague about what scale matters here, please do not read too deeply) are galaxies. So, they will behave similarly to those noninteracting particles from the first example (they do move wrt. each other but slowly "compared to speed of expansion" -- once again sloppy scales).
