Constraining Dark Matter candidates from structure formation

We show that collisional damping of adiabatic primordial fluctuations yields stringent 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. We find that only Dark Matter candidates having cross-sections at decoupling smaller than $10^{-38} cm^2$ with photons and $10^{-40} cm^2$ with neutrinos can explain the observed primordial structures of $10^9$ Solar mass. These are severe constraints which may restrict or exclude even standard WIMP candidates like Supersymmetric particles. When just satisfied, they provide a natural place for Warm Dark Matter candidates. These damping constraints, on the other hand, leave open (less known) regions of parameter space corresponding to particles having rather high interaction rates, with other species than neutrinos and photons.