In my intro cosmology class, we modeled dark matter as a perfect fluid with some classical density field that interacted with everything else until some very short time after the Big Bang and then stopped interacting except by gravity. We set the equation of state to match that of normal matter: if you put 1 gram of dark matter in a box and slowly expand the box to double its volume, you still have 1 gram.

What makes fermionic dark matter be preferred over (massive) bosonic dark matter? I'm not sure why it should make any difference.