Dear people,
the question is simple: Why do we think of particles (or particle like entities) in the first place when talking about Dark Matter?
Even if there are some (and even in some cases better) explanations of the discrepancy we see in galaxy formation and rotation by modifying rules of gravitation on very small and very large scales (esp. Modified Newtonian dynamics MOND), i ask myself why not black holes could be another approach for describing the discrepancies (especially overall distributed primordial black holes, which - if they exist - also should interact with gravity without an electromagnetic observation). What is the key reason to say that Dark Matter must be made of Particles or similar entities?
Thank you.
Careful examination of anisotropy reveals an existence of dark matter in primordial times before decoupling (2008, 2015). At the same time BOSS experiment (2016) does also see existence of dark matter and dark energy in a 3-D map of the sky which is more or less of the same proportion as that which was reported by Planck experiment (2015). So we may conclude that primordial black holes, which were formed at the very initial stages of the formation of the universe, can also play the role of dark matter. There is no reason to believe that dark matter is a single component object. There may be several contributions to the aggregate amount of dark matter. A part can indeed come from primordial black holes whereas another part can be due to exotic elementary particles. A detailed study should include calculations of dark matter relic abundance for possible dark matter components.
http://cerncourier.com/cws/article/cern/65805
http://cerncourier.com/cws/article/cern/60579
http://cerncourier.com/cws/article/cern/33800
Case of black holes is in some sense special. Among all known elementary particles including the Higgs boson, only neutrino can play a role as hot (relativistic) component of dark matter; and this case is now disfavoured. To include an exotic particle as a cold component, one necessarily has to venture beyond the standard model. On the contrary, existence of black holes is predicted by known theories such as Einstein's general theory of relativity. We therefore see that for the case of black holes we need not introduce a new theory.
You may ask why neutrinos are disfavoured as dark matter. The point is that in this case cosmological structures cannot form at the scale of galaxies. For this reason, after publication of COBE results in mid nineties, hot dark matter model lost it's popularity whereas mixed dark matter model (hot+cold) took over as the leading theoretical model, which was again replaced thereafter by a Lambda-CDM model after discovery of accelerated expansion of the universe.
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