Seems like everything in the cosmos is in orbit around something else. Planets orbit stars, moons orbit planets - and dwarf galaxies orbit big galaxies.

This is not news, of course. However, the discovery of four new satellite galaxies of the Milky Way has made the news. This increases the number of the Milky Way's satellites to 20, but the real news is that the discovery is starting to make scientists re-think their ideas about "dark matter."

Just what "dark matter" is has been, so far, maddeningly vague. Scientists agree that it exists, and has something to do with the formation and movement of just about everything in the universe. The most recent idea is that "dark matter" consists of "warm, fast-moving particles." However, the recent discovery of these additional dwarf galaxies in orbit around the Milky Way has challenged this view.

Unlike the Milky Way, which is about 100,000 light years across, these dwarf companions are only a few hundred to a few thousand. They are thought to clump together, eventually forming larger galaxies. By observing these dwarf galaxies, scientists hope to understand this process better.

If the exact number of orbital dwarf galaxies could be determined (the latest discovery suggests there are far more out there than previously thought), it could help scientists to understand more about the nature of "dark matter," which can presently be observed only indirectly - by studying its gravitational effect on light and "normal" matter.

Currently, there are two models of dark matter. The "warm" model suggests that matter moves too quickly to clump together, while "cold" matter moves slowly, easily forming clumps - such as galaxies. Knowing how many dwarf galaxies are out there and how they are distributed would help in distinguishing between the two models. "If there really are 40 to 50 clumps out there, it really starts to look bad for warm dark matter models," says astronomer James Bullock of UC Irvine.