Could light be dark matter? Is it possible that light itself is dark matter?  I am speaking of photons (e.g. visible light, infrared, ultraviolet, etc...).  I realize light is understood to be massless, but it is obvious it at least contains energy because we can see with it (e.g. it energizes the cells in our retinas).  I wonder if light has a very tiny "net mass" (e.g. 0 mass * relativistically infinite speed).  I would think that light at least has a little mass, in proportion to its energy.  For example, take E=mc^2, then m = E/c^2 would describe how much mass it has.  If this is true, light should have a very little gravity too.  Although the effect would seem minimal, light is practically everywhere. Gravity from light would be more concentrated inside galaxies, and even more concentrated in the center of galaxies where there are many stars (like dark matter is).  It would be interesting to run the calculations, assuming light does have gravity, and see if this matches the gravitational observations of dark matter in the universe.  It would be funny and ironic if dark matter really is light.

Edit: Note that it appears light does have gravity as per the discussions here: How does light affect the universe?  If that much is true, I wonder if this is significant enough to account for dark matter?
Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain.

The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is what binds galaxies together. 

Furthermore, we have included everything we can observe (all ordinary matter including photons) when we do the calculations for the amount of gravity there should be. So light is already there. 'Dark matter' is that extra gravity which we cannot account for.