Researchers may have explained why a few tiny galaxies around the Milky Way and the Andromeda galaxies are so rich in dark matter, the invisible stuff that makes up most of the matter in the universe. The key seems to be the bigger, brighter galaxies next door. Simulations indicate that million-degree coronas around these larger galaxies could have scoured away much of the visible gas in their young neighbors while leaving the dark material behind.

Researchers believe that all galaxies large and small should have started out the same—as a ball of dark matter with a disk of visible matter in the center. But some small galaxies, called dwarf spheroidals, are relatively dark for their size; a handful contain roughly 100 times more dark matter per star than the Milky Way and are a million times less luminous. They also tend to cluster around bigger galaxies such as our own. The big question is why.

In new simulations of galaxy formation, Lucio Mayer of the University of Zurich and his colleagues find that, 10 billion years ago, the darkest of today's spheroidals (such as Draco, Ursa Minor and Andromeda IX) were forming around big galaxies from the same mix of visible gas and dark matter, much like planets would form around a star. But they happened to get pulled into orbit around the central galaxy earlier than their counterparts.

Once there, according to the group's simulations, shocks from the central galaxy's gravity, and pressure from the hot corona around it, combined to knock loose most of the smaller galaxies' shimmering gas. Only a few remaining stars studded each blob of dark matter. Ultraviolet radiation, which permeated the universe at the time, would have heated the spheroidals' visible gas, leaving it weakly attracted to the little galaxies and thus easy to scrape away, the group reports in this week's Nature. The model is the first to explain why the spheroidals would be both dark and found near bigger galaxies, says co-author Stelios Kazantzidis of Stanford University.

The researchers "do a good job of simulating all of the relevant physics and setting the orbits" of the dwarfs, says astrophysicist James Bullock of the University of California, Irvine. "They make the case that the Milky Way has very likely stripped the gas (and the 'life') out of many of the dwarf galaxies we see around us."

Kazantzidis says the result may also explain why bigger galaxies have many fewer dwarfs around them than the reigning model of "cold," or slow-moving, dark matter would suggest. Other spheroidals may have received a larger dose of the same gas-stripping process, leaving them even less visible and waiting to be discovered.