A black hole is essentially a ludicrous amount of mass concentrated in an incredibly small space. They're called singularities because gravity becomes measurably infinite at a single point. They suck in everything around them like cosmic vacuum cleaners, and past the event horizon of a black hole nothing can escape, not even light.
What's happening at the center of active galaxy 3C 75? The two bright sources at the center of this composite x-ray (blue)/ radio (pink) image are co-orbiting supermassive black holes powering the giant radio source 3C 75. Surrounded by multimillion degree x-ray emitting gas, and blasting out jets of relativistic particles the supermassive black holes are separated by 25,000 light-years.
European Southern Observatory's Very Large Telescope Interferometer in Chile to observe the dust around the supermassive black hole at the center of the NGC 3783 galaxy, which lies tens of millions of light-years awayin the constellation Centaurus. The black hole, like many at the centers of galaxies, is gorging on a feast of mass that's fallen toward it from the surrounding area. As the dust falls in, it releases powerful radiation that can be spotted from across the universe.