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Do all galaxies spin in the same way? Do we predict an equal proportion of dark matter in all galaxies based on observations? Or is there some variability?

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Yep, pretty much. They all have a flat rotation curve (which makes no sense unless gravity works differently or there is unseen mass), and that rotation correlates to the visible matter in the galaxy (the Tully Fischer relation).

Wouldn't we expect some galaxies to have more dark matter than others? Although, I guess if anything is big enough to converge to an average, a galaxy is.

Apparently they all have just the right amount arranged in the right way to give the same result in each case.

Isn't that weird?

Visually galaxies are quite varied, which implies, to me, they have a lot of variability in visible matter. Why wouldn't their supposed dark matter vary too?

They do! In particular, there are three of them out there that match expectations and thus have little/no dark matter! NGC 1052-DF2 (2018) NGC 1052-DF4 (2019) and AGC 114905 (2022)! Apparently there's another being discussed in a 2026 paper!

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