Well we've some strong indications of what dark matter is and it's not the sort of stuff that would make up things like life forms etc.
Antimatter is like that and there is a (slim) chance that some galaxies and stars and life forms are anti-matter.
But coming back to dark matter thetwo strongest candidates are Machos and Wimps
A Macho is a collective name for thinks like black holes, neutron stars etc anything that doesn't emit light
http://en.wikipedia.org/wiki/Massive_compact_halo_object
I don't think many people still think that Machos are enough to explain the effects of dark matter that we see
Wimps are Weakly interacting massive particles.
http://en.wikipedia.org/wiki/Weakly_Interacting_Massive_Particle
these are subatomic particles that interact so slightly we only see their gravitaional effect.
Take neutrinos - once we thought they were massless - we now know that they do in fact have mass but it's so slight we can't measure it
But there's a lot of them - every photon is associated with an anti-neutrino and that adds up!