Q&A: Dark Matter

Q:
Is it possible that dark matter is not dark at all? According to Einstein's theory of relativity, energy has mass. Although this is a special theory limited to bodies moving in the absence of a gravitational field, the idea that energy has mass should still persist. Light and other rays from the electromagnetic spectrum are seemingly everywhere, but obviously more condensed in galaxies. These rays obviously have energy and possible very small amounts of mass each. Can this ambient energy-mass (if not already included) account for the seeming presence of dark matter??

A:
Einstein's equation linking energy and mass holds in all of spacetime, at all energies. It is true that photons (or light rays) have a small amount of mass associated with them. However, there are about 2 billion photons for every hydrogen nucleus, or proton, in the universe, and their mass equivalent is about one trillionth that of a proton, so the mass equivalent of the photons is about 500 times less than that of the protons. Thus there is not enough mass to explain the mystery of dark matter. Please visit the Chandra field guide to dark matter for more information:
http://chandra.harvard.edu/xray_astro/dark_matter.html