In the cosmological context the current contribution of photons to the energy density of free space is negligible.

However there are entirely theoretical phenomena that occur under extreme radiation-dominated energy densities. Most notable is the Kugelblitz: a black hole formed by concentrating light (or heat) sufficiently dense as to curve spacetime and create an event horizon. This entirely theoretical black hole is one of a class of phenomena proposed by Wheeler in 1955 called "Geons" in his paper of the same title.

A model of these (again theoretical) geons has been extended to a candidate for dark matter called Graviballs. Which is a neat idea, but has no supporting observational evidence that I'm aware of.