Abstract: I will talk about the cosmological constraints on the warmness of keV dark matter, as revealed by measurements of DNeff and the matter power spectrum. With these constraints and considering degenerate fermion system at cosmological scale, the minimal mass bound for fermionic dark matters will be presented. I will then discuss freeze-in dark matter models with various thermal history, e.g. from heavy particle decay, and examine how cold keV dark matter can be upon production.