Geometrical Optics in a Universe with Dominance of Dark Matter

A Friedmann cosmological model, generalized to the epoch of dominance of dark matter, is considered. Its equation of state is chosen in a new, nonstationary form. Basing our approach on the process of light propagation in such a metric, we have found its refractive index, which turns out to be a constant quantity (or more accurately, one that depends on the epoch of the end of dominance of dark matter) and predict a possible effect of a burst of incoming radiation.