The mysterious substance known as dark matter—which makes up six times more of the universe than ordinary, visible matter—may have finally yielded one of its secrets. Astronomers studying the behavior of supermassive black holes, the gigantic beasts lurking at the hearts of most galaxies, have discovered that dark matter seems to have a density limit. As a supermassive black hole sucks in material from the surrounding galaxy, part of that material is dark matter, which is invisible except for its gravitational pull. Astronomers know that galaxies need the extra gravity from dark matter to keep their constituent stars from flying off into space. But if too much dark matter got pulled into supermassive black holes, there wouldn't be enough to hold galaxies together. Researchers calculate in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, that the maximum density for dark matter must be seven times the mass of the sun per cubic light-year of space. Any more than that amount, and supermassive black holes would have grown so large that they would have devoured giant swaths of material from the hearts of galaxies, possibly obliterating them entirely.