Main Question or Discussion Point
Just to clarify, I am NOT talking about the Dark Energy, I am talking about the Dark Matter
It doesn't interact with itself either. At least not much.No, it is pressureless because it does not interact with our matter.
I can imagine a 'dark radiation', which is 'pressureless', but has w=1/3
But yes, probably DM now is a cold gas with w=0
Not sure about the early ages of our Universe still
Well, it's also because it's currently at low density. Normal matter experiences quite a lot of pressure at higher densities. But on cosmic scales this effect is completely negligible.Yep. Regular matter has w ~ 0 as well - it's because it has in essence no kinetic energy compared to its total energy. It doesn't really matter whether it interacts or not. Relativistic matter would have an equation of state parameter such that 0 < w < 1/3, with the exact value depending on the kinetic energy. It approaches 1/3 as its' kinetic energy becomes much greater than its rest energy.
Er, well, no. The velocities of these particles within galaxies and galaxy clusters is actually irrelevant as far as this determination is concerned. The dark matter particles get heated by their fall into those gravitational potential wells. Dark matter particles that haven't fallen into such wells are actually probably much cooler than the CMB, because the photons get lots of energy dumped into them by various processes, but as the dark matter loses its ability to interact early-on, it doesn't get this extra energy.So as DM is captured around the Galaxies, DM particles have typical velocity in a range of 100km/s. This is equialent to a high temperature if their mass is = mass of proton. But it is logical to assume that DM has the same temperature as other relic sorts of matter, about 3K. Then DM particles must be very light - even lighter then electrons.