The fun question to ask is why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them. That dark matter is organising itself to produce the same outcome over many different circumstances!

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marcosdumay an hour ago | parent | next [-]

> why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them

They don't. You can read exactly this article for 3 examples.

Modified Newtonian theories are anything but simple.

	▲	MattPalmer1086 11 minutes ago \| parent \| next [-]
		If you read the rebuttal by Professor McGaugh, you can include those 3 as perfectly good examples of MOND too. You do make a good point though - astronomy and astrophysics is hard. There are often many ways of interpreting the observations we have. They can be misleading and take careful interpretation, particularly when they seem unusual in some way.
	▲	10 minutes ago \| parent \| prev [-]
		[deleted]

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Buttons840 2 hours ago | parent | prev | next [-]

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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MattPalmer1086 2 hours ago | parent [-]

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).

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Buttons840 an hour ago | parent [-]

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.

	▲	MattPalmer1086 4 minutes ago \| parent [-]
		Apparently they all have just the right amount arranged in the right way to give the same result in each case.

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thrance an hour ago | parent | prev [-]

Nope, there are galaxies that spin exactly as our current model predicts they should, while others are way off, as if there was something unevenly distributed across them. There's no simple modification to our theories that can account for these discrepancies.