A recent paper is causing a stir among cosmologists and bloggers because it appears to present evidence that dark matter may not exist. The authors’ observations reveal a strong correlation between the gravitational acceleration observed to be acting on stars in galaxies and the gravitational acceleration inferred from the distribution of luminous (baryonic) matter. If dark matter is a particle, as almost everyone has been assuming, it can’t explain this behavior. Ethan Siegel at Starts with a Bang comments on the situation here and Sabine Hossenfelder at Backreaction discusses it here.
The authors of the paper, who are believers in modified gravity or MOND (MOdified Newtonian Dynamics), suggest that the observed behavior might indicate that something is missing from the current paradigm for galaxy formation, that something like MOND is going on, or that new dark sector physics is responsible, for example, perhaps dark matter is not a particle but behaves more like a fluid.
Of course, as readers of this blog should know, dark matter is not a particle, but a remnant of inflation. The oscillations of spacetime that are the end product of inflation don’t decay completely to form particles. Only about 16% of the energy decays to form baryonic matter. The rest of the energy is still there in spacetime. It’s not a particle but it is energy and it has gravitational effects. A number of mainstream cosmologists have published papers suggesting this possibility (see here, for example), but most of their colleagues reject it (see here). One group of authors has published a paper showing how spacetime energy without matter could mimic dark matter, but they rejected their own idea because they couldn’t see how such a thing could happen.
In fact, it can’t happen in the current paradigm for particle physics and cosmology because the spacetime model is all wrong. Actually, it’s nonexistent. If you have the right spacetime model, that is, the model that’s the principal subject of this blog, none of this stuff is very mysterious. See here for a discussion of inflation and dark matter.