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If gravity slows the effects of time, then empty space will see greater expansion than space inhabited by massive objects. So the space within a galaxy will be expanding more slowly than the space without. Maybe the effect is minimal, but in cosmological size and time frames, the effect could be significant? (A presumption I'm making)

Space is also expanding faster than light

There's also the missing mass problem. Scientists haven't been able to fully explain why galaxies don't fly apart because they're moving faster than they should be able to, though there are some explanations and also this

But I haven't seen anything that has said any of those answers have completely solved the issue. Is it possible that empty space, expanding faster than massive space, could exert positive 'pressure' on galaxies?

Sorry I'm just a layman but I haven't seen anyone discussing this specifically.

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  • $\begingroup$ The 1st sentence of the question provides the best explanation I've ever seen for the standard view that astronomical bodies (like galaxies) remain stationary within expanding space. $\endgroup$
    – Edouard
    Commented Jun 4, 2022 at 16:18
  • $\begingroup$ Gravitationally bound objects (like galaxies) do not participate in the expansion of space, and even if they did, the effect on galactic dynamics would be negligible. $\endgroup$
    – D. Halsey
    Commented Jun 5, 2022 at 17:32
  • $\begingroup$ Space expanding faster than the speed of light only occurs over distance scales several orders of magnitude larger than any galactic radii. $\endgroup$
    – D. Halsey
    Commented Jun 5, 2022 at 17:33

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Just to deal with these problems physicists introduced the term dark matter. In the first question the expansion rate of galaxy as you said is slowed down by celestial objects' gravity. But Hubble and ESA measured that the mass of stars and gases present in our galaxy accounting only for 16% right. So we can assume that the remaining 84% mass is that of the dark matter which also holds together the galaxy from splitting apart while they move faster than what they are actually able to. We can also say that dark matter is evenly distributed all over the universe so that the gravity exerted by them is even all over the universe. Only the 16% of gaseous and stellar mass creates a difference in the rate of expansion I hope so. The thing you said about the pressure exerted on galaxy is somewhat confusing because that (i think) might not actually affect the galaxy.

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  • $\begingroup$ I think the "pressure" that the OP had in mind might basically be the difference between the spatial expansion rate and whatever expansion rate a galaxy might have. But, since he mentioned time dilation, the possibilities involved may be greatly affected by confusion between special relativity and general relativity: This confusion is discussed a lot by Lineweaver and Davis, at arxiv.org/abs/astro-ph/0310808 , and (more simply) in an article that they wrote for a magazine, which I believe was "Scientific American". $\endgroup$
    – Edouard
    Commented Jun 4, 2022 at 16:30
  • $\begingroup$ You're right, I removed the time dilation tag. Not what I meant to add. I'm aware of dark energy/matter but those are just placeholders for the true reason we're missing all that matter/energy. I'm positing a thought about one possibility. If I'm wrong I'd love to hear why, I just haven't heard any discussion about whether space expands more in empty space than massive space and the effects that might have on the cosmos. $\endgroup$
    – Josh Kroslowitz
    Commented Jun 4, 2022 at 19:04

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