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From New Atlas:
Some of the strongest evidence for dark matter to date has been discovered – and ironically, that's thanks to its absence. In a pair of studies published this week, astronomers have shed new light on dark matter through close observation of a galaxy previously found to have very little of the stuff, while the same team found a new example of a similar oddball galaxy.
It's generally believed that galaxies are held together through the gravitational influence of clumps of dark matter, so to find a galaxy with little to no dark matter was a surprise. And while it might sound like a strike against the theory, it actually ends up supporting it.
A Second Galaxy Missing Dark Matter in the NGC 1052 Group (DOI: 10.3847/2041-8213/ab0d92) (DX)
Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC1052-DF2 (DOI: 10.3847/2041-8213/ab0e8c) (DX)
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @09:22AM (3 children)
Sorry, I thought you had some idea of what you are talking about. MOND has predicted every galactic rotation curve discovered since 1983. Dark matter has been post hoc fit to every single one, just exactly enough to match the MOND prediction.
(Score: 1) by khallow on Tuesday April 02 2019, @10:14AM (2 children)
So has dark matter based theories. Epicycles are a wonderful thing, until you get too many of them.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @10:23AM (1 child)
No they have not. They have never predicted a single one because it is impossible. All black matter explanations are post-hoc. Every single one.
And MOND doesn't have epicycles. It is a simple relationship where you plug in values for the surrounding mass. The only trouble is getting good estimates of those values. There is nothing similar to epicycles that are added ad hoc.
Dark matter on the other hand, is literally 3D epicycles. It is invisible spheroids of stuff added in after the fact to make the predictions work.
(Score: 1) by khallow on Tuesday April 02 2019, @10:44AM
To the contrary, small object near a large object (where the weakly interacting matter can be drawn off by gravitation interference) is one such place. It predicts the existence of galaxies without much in the way of dark matter. The mechanism of creation conveniently falls under the external field effect (EFE) assumption.
Two have already been mentioned. The initial weak gravity deviation from Newtonian mechanics. And the EFE assumption. Neither has physical explanation. It just fits data, just like the dark matter assumption does.
In other words, "simple relationship where you plug in values for the surrounding mass". We already know the universe has priors since distribution of matter follows no simple relation on any scale we can observe. So dark matter priors is not even a little bit of a stretch.