(published on 03/12/07)
When physicsts use "light" in this general topic, we usually mean any type of electromagnetic radiation, not just the little visible slice of the spectrum. It turns out that no form of electromagnetic radiation can account for either dark matter or dark energy.
Dark matter is probably not all that different from more familiar forms of matter. It certainly isn't electromagnetic radiation itself, since that doesn't clump up gravitationally with other matter, and dark matter does. It's called dark precisely because it has no electromagnetic interactions. That's why a cloud of it can move right through another galaxy, while the ordinary matter interacts via electromagnetic forces.
Electromagnetic radiation also can't be dark energy. Classical EM radiation doesn't cause an accelerating expansion, but that's what dark energy was invented to try to explain. In principle the zero-temperature quantum EM background would cause an accelerating expansion, but the effect would be way too big. (see the discussion of Casimir in Cosmology on http://van.physics.illinois.edu/qa/listing.php?id=22970) So we need some other sort of understanding of the whole acceleration issue.
(published on 06/16/13)