Our Milky Way galaxy is haunted. The vast gulf of space between the stars is plied by the dead, burned-out and crushed remnants of once glorious stars. These black holes cannot be directly seen because their intense gravity swallows light. Like legendary wandering ghosts, their presence can only be deduced by seeing how they affect the environment around them. Imagine crushing the mass of a fleet of battleships into something no bigger than a baseball. That only begins to describe the infinite density locked away into a black hole left over from a stellar explosion. The black hole is typically...