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As the universe expands, why don't galaxies get stretched out?

There is enough matter in a galaxy, that locally the expansion of the universe is stopped. You can think of this as the gravity of the galaxy holding it together, but really it's more fundamental than that. The rate of the expansion of the universe depends on the amount of matter (and dark energy) in the universe. If you just consider a tiny fraction of the universe which just includes a galaxy and total the matter in that region, it's more than enough to have already stopped the expansion in that region.

October 2004, Karen Masters (more by Karen Masters) (Like this Answer)

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