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] <>

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.

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