Constraining dark matter candidates from structure formation

We show that collisional damping of adiabatic primordial fluctuations yields constraints on the possible range of mass and interaction rates of dark matter particles. Our analysis relies on a general classification of dark matter candidates, that we establish independently of any specific particle theory or model. It holds for very weakly as well as very strongly interacting particles. From a relation between the collisional damping scale and the dark matter interaction rate, we find that dark matter candidates must have 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 warm dark matter candidates whose astrophysical relevance is worth to be explored. They also leave open less known regions of parameter space corresponding to particles having much higher interaction rates with other species than neutrinos and photons. (C) 2001 Published by Elsevier Science BN.