General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.

Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.

However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.

This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.

MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.

But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.

[1]: https://pirsa.org/26030070

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NewEntryHN 3 hours ago | parent [-]

Why is the article titled "Newton's law of gravity passes its biggest test" if it doesn't explain the movement more than MOND?

	▲	magicalhippo 2 hours ago \| parent [-]
		Other way around. Newtonian dynamics explains the data very well, MOND did not. In particular, Newtons law of gravity says the effect of gravity falls off as 1/r^2 where r is the distance from the mass. MOND modifies the standard equations so that gravity starts like 1/r^2 when r is small, and acceleration is large, but for greater distances, when the acceleration is low, instead falls off like 1/r. MOND explains the movement of the stars in (most) galaxies very well. However this result showed that MOND was not consistent with the motion of the galaxies in the cluster. On the other hand the motion was consistent with plain Newtonian dynamics. Hence Newtons law of gravity (and by extension GR) passed the test.