Dark matter is 85% of matter. You are talking about such an obscene amount of light that it would be very very easily detected. Space isn't empty, and the space around galaxies isn't empty. You are talking about there being so many supernovas worth of light out there between galaxies that it's just ... I dont even know. The only way it would make sense is if the huge amount of light out there didn't interact with regular matter in any sort of way. Otherwise it would be scorching the earth with a fury of a trillion gamma ray bursts. Hmm, a form of matter that is everywhere but doesn't I interact with normal matter in any sort of currently detectable way ... I wonder what that would be.