  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.
