In the center of our galaxy lies a huge black hole, so big we call it supermassive: It has the mass of four million times our Sun's. Called Sgr A* (literally spoken as "Sagittarius A star," or "Saj A star"), the evidence for it is overwhelming; we can see stars orbiting it, their positions changing over the course of years and in some cases months. The math of orbital mechanics tell us they are enthralled to an object with several million times the mass of a star, but if there were that many stars there they'd glow with fierce light. We see essentially nothing, so a black hole it must be.
But… is it alone? Could there be a second black hole there, one less massive but still dizzyingly hefty, orbiting Sgr A* closely?