The mathematics of physics and astronomy are way over my head and I can’t speak with expertise on the topic; I’m an amateur, not an auteur. But I’m curious and follow news stories on it. A new study was published in November 2020 issue of The Astrophysical Journal (PDF) that addresses a problem that Dark Matter hasn’t answered:
The strong equivalence principle (SEP) distinguishes general relativity (GR) from other viable theories of gravity. The SEP demands that the internal dynamics of a self-gravitating system under freefall in an external gravitational field should not depend on the external field strength. We test the SEP by investigating the external field effect (EFE) in Milgromian dynamics (MOND), proposed as an alternative to dark matter in interpreting galactic kinematics. We report a detection of this EFE using galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample together with estimates of the large-scale external gravitational field from an all-sky galaxy catalog.
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In this paper we provide observational evidence for the existence of the EFE (or a phenomenon akin to it) predicted by MOND modified gravity (Bekenstein & Milgrom 1984). We use accurate rotation curves and mass models from the SPARC database (Lelli et al. 2016) and detect the EFE in three separate ways:
[. . .]
Our results suggest a breakdown of the SEP: the internal dynamics of a gravitational system in freefall is affected by a uniform external gravitational field. This sheds new light on the dark-matter problem and paves the way for relativistic theories of modified gravity in the weak-field regime of gravity g lesssim 10−10 m s−2.
The effect was observed on only 153 galaxies. Based on the opening statement, it doesn’t look like they were being selective (read: cherry picking) but rather there weren’t many testable cases. More is always better, and I’m sure they’re continuing to look for more.
My view has long been the same as what Dr. Becky Smethurst (Dr. Becky, as she describes herself on youtube) says in her video, that if we do find an answer on how the universe works, a Unified Field Theory, it’s likely to explain or incorporate both MOND and Dark Matter, much the same way that Andrew Wiles’s proof of Fermat’s Last Theorem connected multiple theories and conjectures.