Things like asteroids, galactic dust and the like are already accounted for in the baryonic (ie ordinary) matter. We can estimate it for example measuring the absorption of different wavelengths of light, or extrapolating the local abundance of asteroids. There are theories like the MACHO that propose that we are missing some, but in general it is understood they can only account for a tiny fraction of the missing mass.

The predominant hypothesis is that dark matter is composed by some unidentified particles, that have the same thermodynamic properties as usual matter (basically that their energy is proportional to the volume), but that don't interact (or interact very weakly) with normal matter.