Louis J Sheehan 238210.028

Louis J Sheehan The matter that makes up everything we can see or touch, either on Earth or beyond, is exceedingly rare, cosmically speaking. Most of the material in the universe is something called dark matter, mysterious stuff that doesn’t emit or reflect light and doesn’t interact with what we think of as ordinary matter. It reveals its presence only by its gravitational effects, guiding the evolution of the early universe and still affecting the motion of galaxies. Earth-based experiments have attempted to detect dark matter particles, but so far they have drawn a blank.

Astronomers, however, have had a better year, continuing to find evidence of the crucial role dark matter plays in shaping the visible cosmos. Thanks to about a thousand hours of observation by the Hubble Space Telescope, scientists have compiled a dark matter map of a tiny slice of the sky, about two square degrees of the entire sky’s 40,000-square-degree span. The map, which was published in the journal Nature last January, confirmed a central prediction of modern astrophysics: Galaxies formed in, and remain bound to, enormous clouds of dark matter.

In the early universe, astronomers believe, dark matter provided the gravitational scaffolding on which ordinary matter coalesced and grew into galaxies. According to these dark matter theories, as the visible galaxies formed, some of the matter surrounding them should have clumped together into hundreds of small satellite galaxies, most of which should survive today. But the observed number of satellite galaxies is only a fraction of what the theory predicts. “We should see about a hundred to a thousand, but up to 2005, there were only 12,” says Marla Geha, an astrophysicist at Yale University. Astronomers call it the missing satellite problem. HTTP://LOUIS-J-SHEEHAN.US