Black holes are objects so dense and with a gravitational potential so strong that nothing, not even light, can escape the pull if it ventures too close. Black holes are invisible, yet the gas and dust falling into a black hole are heated to high temperatures and glow furiously. Scientists agree that there are at least two classes of black holes. Stellar black holes, with a mass of up to about ten suns, are the remains of massive stars whose cores have imploded. Supermassive black holes contain the mass of millions to billions of suns confined to a region about the size of our solar system. These monstrous objects likely form from immense gas clouds and are thought to reside in the cores of most galaxies. Scientists are not in agreement over the existence of intermediate-mass black holes, however, which seem to harbor the mass of hundreds to tens of thousands of suns. Now, a team led by Jon Miller of the Harvard-Smithsonian Center for Astrophysics zeroed in on gas very close to two suspected “intermediate-mass” black holes – material that would soon take that final plunge. Using the European Space Agency’s XMM-Newton satellite, the scientists precisely measured the temperature of this gas and obtained the most accurate mass measurement of the black hole systems to date. “Evidence is mounting that these elusive intermediate-mass black holes may really exist,” says Miller. “The mystery, really, is how they can exist.”