Dark matter is suppose to form large halos of unknown yet type of particle with a specific density density profile. Does the dark matter halo can be viewed as a fluid of particles with density and non-zero pressure? Which kind of equation of state one could expect for the dark matter fluid?
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The answer to your first question, about considering dark matter as a fluid, is that when you are modelling the different components of the universe ('common' matter and dark matter, radiation and dark energy) you work with them as they were perfect fluids, that means no viscosity nor heat conduction.
About the equation of state and the pressure related with this component of the universe I have to say that all these components are studied with barotropic equations of state (the pressure of the fluid only depends on its density):
$$ p=\omega \rho $$
with $\omega$ a constant that depends on the component. The pressure associated to the dark matter is non-zero but it's so small that is possible to consider it as zero.
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$\begingroup$ I was reading other questions and since the dark matter is collisionless, then one should expect $P=0$, however, in Nature anything is perfectly 0, so, are there limits for $P?$ who has measured that? $\endgroup$ Commented Aug 14, 2018 at 9:34
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1$\begingroup$ One property that is associated with dark matter is that it has to be cold (CDM), that means that most probably is formed by non relativistic particles. So when people model these different components of the universe as barotropic fluids the distinguish relativistic and non relativistics components with different values of the constant $\omega$ ( $\omega = 1\3$ for ultrarrelativistic components and $\omega = 0$ for non relativistic ). These values are the limit situations, you can find possibilities in betwn $0<=\omega <= 1/3$. Smthng funny is that to describe dark energy you need $/omega=-1$. $\endgroup$ Commented Aug 14, 2018 at 11:30