Question: After a star lives and dies, I assume virtually all of its mass would be photons. If enough stars have already lived and died, couldn’t there be enough photon energy out there to account for all the "missing mass" (=dark matter) in the universe?

And if there were enough photons to account for all the missing mass, what would it look like to us?

Answer: 
As a general rule, zero mass particles which travel with the velocity of light are not good for dark matter, because [dark matter concentrates](http://en.wikipedia.org/wiki/Dark_matter_halo) around gravitational attractors. It has to be particles with some mass that can be at rest in order to stay around a galactic center from the beginning. In addition they have to be controlled by weak interactions, if they decay, because the dark matter halo is stable for long periods.

Maybe I should add that very cool photons from the beginning of the formation of the observed universe exist and have been detected as [Comsmic Microwave Background](http://en.wikipedia.org/wiki/Microwave_background_radiation) radiation, very low frequency photons, uniformly distributed in the cosmos.
It is accepted that the early state of the universe was mainly radiation, then we had expansion; radiation cooled down as a result, and condensed into matter. The pressure of a photon/radiation gas is positive and given by an equation that resembles the universal gas law; PV=.9 NkT (<https://en.wikipedia.org/wiki/Photon_gas>). 
So by logic, the total(equivalent) mass or the total condensed energy in the universe, must be less than the total radiation at the start. If also the condensation process involved only a small portion of that radiation, then surely we should have plenty of radiation left uncondensed- and could easily be much more than that of all matter. This remaining gas is continuing its expansion of course- today and in the future.Since momentum levels follow energy levels for photons, there must be plenty of momentum around too- due to radiation that is exerting pressure on the various matter objects, producing some expected and may be not so expected situations. If we accept this point of view, one can do detail calculations to get the full picture. The rate of expansion could be one useful result to compare with present data.