I have read this question:
The only way you can do this is to remove kinetic energy from the system. With normal matter this is done through electromagnetic interactions, which turn the kinetic energy of normal matter (protons, electrons etc.) into photons, which then escape from the system. Since these kinds of interactions do not occur for dark matter (by definition), then there is no way to get rid of kinetic energy and so the dark matter remains as a large "halo" around gravitationally clumping ordinary matter.
And this one:
A significant portion of the dark matter is known to be gravitationally bound to galaxies and relativistic velocities are far about the escape velocity and the stuff you propose would not remain bound.
I assume that its speed with respect to the Sun will have a distribution with an rms of a few 100 km/s.
In general particles created at the time of Big Bang are at non relativistic velocities at present .
The first one says that dark matter cannot lose kinetic energy through interactions. As far as I understand, most ordinary matter moves at non-relativistic speeds around us because these are able to lose kinetic energy through interactions and thus we see objects made of these particles in the observable universe to be mostly non-relativistic (except for example neutrinos).
Now if dark matter cannot lose kinetic energy, then shouldn't it have most of its kinetic energy from the Big Bang and travel at relativistic speeds? The second one says dark matter is definitely non-relativistic.
If dark matter can't lose kinetic energy, then why is it not traveling at relativistic speeds?