Lurking in the depths of a galaxy, hidden from human eyes, lie millions of monsters. They could swallow you up in an instant, sealing you off from the outside world and devouring you atom by atom.  This sounds like your typical Hollywood monster movie, but with millions of black holes hidden throughout the galaxy, its more real than you might think.  Supermassive black holes, the largest ones that reside at the centers of galaxies, are much easier to see.  They are devouring gas and dust rapidly, resulting in bright emission across the electromagnetic spectrum, especially in x-rays.  For many galaxies,...