DM particles: how warm they can be?
  One of important questions concerning particles which compose the Dark Matter (DM) is their average speed. We consider the model of relativistic weakly interacting massive particles and try to impose an upper bound on their actual and past warmness through the analysis of density perturbations and comparison with the LSS data. It is assumed that the DM can be described by the recently invented model of reduced relativistic gas (RRG). The equation of state of the RRG model is closely reproducing the one of the Maxwell distribution, while being much simpler. This advantage of the RRG model makes our analysis very efficient. As a result we arrive at the rigid and model-independent bound for the DM warmness without using the standard (much more sophisticated) approach based on the Einstein-Boltzmann system of equations. 
Julio C. Fabris, Ilya L. Shapiro, Flavia Sobreira