At the center of the Milky Way is a supermassive black hole, 4.3 million times bigger than the sun, known as Sagittarius A*. Until recently, it was not clear how much of the matter at the heart of the galaxy was Sagittarius A*. Astronomers measured the velocities of four distant stars around the black hole. The movement of the stars indicates the mass at the galaxy's center is composed almost entirely of matter from Sagittarius A*, leaving little room for stars, other black holes, interstellar dust and gas, or dark matter.