2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/227398We construct simple and useful approximation for the relativistic gas of massive particles. The equation of state is given by an elementary function and admits analytic solution of the Friedmann equation, including more complex cases when the relativistic gas of massive particles is considered together with radiation or with dominating cosmological constant. The model of relativistic gas may be interesting for the description of primordial Universe, especially as a candidate for the role of a Dark Matter.LaTeX, 12 pages, 4 figures. Several references on supersymmetric Dark Matter and one illustrative plot added. Final version accepted in Modern Physics Letters AGeneral Relativity and Quantum Cosmologytext