https://backreaction.blogspot.com/2017/10/new-gravitational-wave-detection-with.html
New gravitational wave detection with optical counterpart rules out some dark matter alternatives Sibel Boran, Shantanu Desai, Emre Kahya, Richard Woodard arXiv:1710.06168 [astro-ph.HE] <https://arxiv.org/abs/1710.06168> Prof. Pierre Sikivie: "It has long been known that axions produced by vacuum realignment during the QCD phase transition in the early universe form a cold degenerate Bose gas and are a candidate for the dark matter. More recently it was found that dark matter axions thermalize through their gravitational self-interactions and form a Bose-Einstein condensate (BEC). On time scales long compared to their rethermalization time scale, almost all the axions go to the lowest energy state available to them. In this behaviour they differ from the other dark matter candidates. Axions accreting onto a galactic halo fall in with net overall rotation because almost all go to the lowest energy available state for given angular momentum. In contrast, the other proposed forms of dark matter accrete onto galactic halos with an irrotational velocity field. The inner caustics are different in the two cases. I'll argue that the dark matter is axions because there is observational evidence for the type of inner caustic produced by, and only by, an axion BEC." There is dark matter theory that shows evidence of BEC formation on the galactic scale. If you need to get to superfluidity, the axion BEC is what is required. Also related to Scalar field dark matter "The dark matter can be modeled as a scalar field using two fitted parameters, mass and self-interaction. In this picture the dark matter consists of an ultralight particle with a mass of O(10e−22) eV when there is no self-interaction. If there is a self-interaction a wider mass range is allowed. The uncertainty in position of a particle is larger than its Compton wavelength, and for some reasonable estimates of particle mass and density of dark matter there is no point talking about the individual particle's position and momentum. The dark matter is more like a wave than a particle, and the galactic halos are giant systems of condensed bose liquid, possibly superfluid. The dark matter can be described as a Bose–Einstein condensate of the ultralight quanta of the field and as boson stars. The enormous Compton wavelength of these particles prevents structure formation on small subgalactic scales, which is a major problem in traditional cold dark matter models. This dark matter model is also known as BEC dark matter or wave dark matter. Fuzzy dark matter and ultra-light axion are examples of scalar field dark matter." An axion BEC on the galactic scale meets the need for the dark matter particle to produce superfluid effects, and in certain approximations, behaves like modified gravity.
