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: 
There is a simple argument why photons emitted by stars can't be dark matter, and that's because there is about ten times more dark matter than normal matter. If all the stars created at the Big Bang had turned into photons there still wouldn't be enough of them.

You might argue that maybe more normal matter than we think was created during the Big Bang, but the theory of [Big Bang Nucleosynthesis](http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis) places a [limit](http://cosmology.berkeley.edu/Education/CosmologyEssays/BBN.html) on how much normal matter was created, and this limit is four times smaller than the amount of dark matter. The dark matter has to be something *odd*.

If you're interested in more info [this paper](http://arxiv.org/abs/0906.2087) is a good review, though harder going than the answers here!
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.