The cosmological effects of cold DM are almost opposite to the hot DM effects. Given that cold DM promotes the growth of large scale structures, it is often believed to be composed of Weakly interacting massive particles (WIMPs). Conversely hot DM suffers of free-streaming for most of the history of the Universe, washing-out the formation of small scales. In other words, the mass of hot DM particles is too small to produce the observed gravitationally bounded objects in the Universe. For that reason, the hot DM abundance is constrained by Cosmology to less than one percent of the Universe contents.