Dark systems
It seems as though there is an assumption that the evolution of the galaxy forms a direct line from dust to stars with the left over dust (around those stars) forming planets and then the leftover dust that didn't make it into the star or planet forms asteroids and comets.
I propose that there is another scenario that not only happens but happens far more often. Dust congeals into balls that don't make it to star size but in fact become dark stars with masses below that of brown dwarfs each with it's own attendant planets and planetesimals. These so called dark-stars are just that - failed stars, they don't ignite. Creating in effect dark systems, not just dark stars.
Is it not possible, that the ‘missing matter’ is made up of dark systems that out number normal systems 10-1.
MACHOs
The idea that you are suggesting has actually been around for quite a bit. Dark (or dim) brown dwarfs or planets are some of the MACHO candidates for the existence of dark matter in the universe, and they have been actively sought (through gravitational microlensing surveys) for at least a decade.
The results from those surveys though, is that only 25-50% of dark matter can be accounted for in this way.
I also have my reservations
I also have my reservations about dark matter. To bolster your point, this autumn, Chandra recently found "orphan" stars & gas in a sort of thin tail, thousands of light years away from the "parent" galaxy (see the complete press release here). I think we're going to find more & more "regular matter" the longer we look, until we find that we won't need any more of the dark stuff, except when it's coffee!