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.
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!