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 ≲10−33mdm1MeVcm2 with photons and ≲10−37mdm1MeVcm2 with neutrinos, to explain the observed primordial structures of 109M⊙. 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.