Nearly Invisible Galaxy Found Orbiting Milky Way

Astronomers have identified the least luminous galaxy known, but it's surprisingly massive.

The reason: It is loaded with invisible matter. Dark matter is mysterious, unseen stuff that permeates the universe. Astronomers know it's there because of the gravity it creates. Without invoking dark matter, theories can't explain how galaxies stay together. The galaxy, called Segue 1, is one of about two dozen small satellite galaxies orbiting our own Milky Way galaxy. A separate study last month, reported in the journal Science, found that all the known satellite galaxies are loaded with dark matter.

But among them, Segue 1 is special. It is a billion times less bright than the Milky Way. Yet it's nearly a thousand times more massive than its star light would suggest. ... "Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass," said study leader Marla Geha, an assistant professor of astronomy at Yale University. -fox