Dark Matter: The Situation Has Changed
It&#x27;s a bit of the same from Sabine. Dark matter hypothesis allowed us to simulate a vast range of processes in the Universe, from large scale cosmological simulations, predicting clustering of galaxies, cosmic microwave background to smaller scale simulations of galaxy formation and individual galaxies. The hypothesis have been extremely successful and predictive. It is also true, that some of the systems we&#x27;ve been discovering are a bit harder to explain within our Cold Dark matter framework, requiring  changing some things about how galaxies form (i.e. making star formation less efficient in small galaxies, introducing feedback effects from accreting supermassive black hols and things like that). I (and many astrophysicists) do not see that as a problem or evidence that dark matter needs to be abandoned, but instead see that as a refinement to the theory.<p>Regarding modified gravity theories, it&#x27;s true that you can mimic some of the dark matter behaviour by the modification of gravity laws. Also if you can add some kind of global scalar field you can similarly mimic dark matter behaviour. The problem however is that the modified gravity theories  (at this point) do not allow the same range of simulations that cold dark matter(CDM) simulations can do (for various reasons, such as many of those theories do not have a general relativity formulation). Because these theories can&#x27;t make as many&#x2F;as varied simulations as CDM ones, they  are not as tested as regular CDM and much less predictive.  Therefore based on that fact alone IMO these theories are less useful.<p>It is also true that if we continue for many years searching for the dark matter particle and will still be unable to find it, maybe we will need to refocus our attention on alternative gravity theories, but the problem is that so far the part of parameter space where we looked for the dark matter particle is still pretty small and there is no strong reason to think that dark matter should have been there.