Quasars, the brightest objects we're aware of, are powered by the supermassive black holes that are thought to reside at the center of every galaxy. But many galaxies fail to feed their black holes enough matter, leading to a body that's quiet and difficult to detect. Our own galaxy's central black hole, called Sgr A*, falls into the latter category. We can detect it at wavelengths up to the X-ray range, but it's dim enough that we'd have a hard time spotting it if it weren't so close.
That may be about to change, however. Astronomers have spotted a cloud of gas with a mass about three times that of Earth that's on a trajectory that will have it pass close to Sgr A* in 2013. When it does, it may feed matter into the black hole's accretion disk, powering a sudden surge in Sgr A*'s output.