Dark matter halos — the huge, invisible cocoons of mass that envelop entire galaxies and account for most of the matter in the universe — may not be so dark after all.
Astronomers have long disagreed about why they see more infrared light in the universe than is emitted by known galaxies. What shows up in images is neither stars nor galaxies, but mysterious smatterings of light known as fluctuations.
Some say these come from very distant unknown galaxies; others, that they're from much closer but unknown galaxies; and now there's a third theory.
The idea that the fluctuations are from very distant galaxiesis nowhere close to being supported by data from NASA's Spitzer Space Telescope, says Edward Wright, a UCLA professor of physics and astronomy.
"The idea of not-so-far-away faint galaxies is better, but still not right," he adds. "It's off by a factor of about 10; the 'distant galaxies' hypothesis is off by a factor of about 1,000."
His team beleives that the small number of stars that were boosted off to the edges of space during violent collisions and mergers of galaxies may be the explanation.
"Galaxies exist in dark matter halos that are much bigger than the galaxies; when galaxies form and merge together, the dark matter halo gets larger and the stars and gas sink to the middle of the halo," he says.
"What we're saying is one star in a thousand does not do that, and instead gets distributed like dark matter. You can't see the dark matter very well, but we are proposing that it actually has a few stars in it — only one-tenth of 1 percent of the number of stars in the bright part of the galaxy."
In large clusters of galaxies, astronomers have found much higher percentages of intra-halo light, as large as 20 percent.
"If we can really understand the origin of the infrared background, we can understand when all of the light in the universe was produced and how much was produced," says Wright.