Constraining dark matter candidates from structure formation

We show that collisional damping of adiabatic primordial fluctuations yield s constraints on the possible range of mass and interaction rates of dark m atter particles. Our analysis relies on a general classification of dark ma tter candidates, that we establish independently of any specific particle t heory or model. It holds for very weakly as well as very strongly interacti ng particles. From a relation between the collisional damping scale and the dark matter interaction rate, we find that dark matter candidates must hav e cross-sections at decoupling less than or similar to 10(-33) m(dm)/1 MeV cm(2) with photons and and less than or similar to 10(-37) m(dm)/1 MeV cm(2 ) with neutrinos, to explain the observed primordial structures of 10(9) M- circle dot. When marginally satisfied, these damping constraints provide wa rm dark matter candidates whose astrophysical relevance is worth to be expl ored. They also leave open less known regions of parameter space correspond ing to particles having much higher interaction rates with other species th an neutrinos and photons. (C) 2001 Published by Elsevier Science BN.