Is Light the Missing Dark Matter?

Could the weight of light account for the missing "dark matter' in the universe? If all the light that ever was still exists (if it didn't, where did it go?) and it is just moving in time from one place to someplace else on an infinite line of time, isn't that a lot of matter over the course of the beginning of the universe? We can only see a small part of it in our singular place on the timeline of light. Perhaps the universe is actually completely ablaze with light.
- Sue Charles (age 56)
Newburyport

That's a cool idea but there are a couple of reasons why it can't work. 

First, the dark matter clumps in with galaxies, held in by gravity. It's those gravity effects that make us know it's there. Light would (and does) escape from galaxies without clumping. In order to contribute to clumping dark matter, particles need some minimum rest mass, but light has no rest mass at all.

Second, there just isn't enough light around to contribute much mass even if you ignore whether it clumps. We've discussed that here:https://van.physics.illinois.edu/qa/listing.php?id=14672.

Mike W.

(published on 02/22/2015)