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!
Photons are easily detectable. We can count how many photons are there at any distance of us by just counting the photons reaching us from there. It is impossible that the hidden photons ramble the whole universe but mysteriously avoid us.