Oops! Andromeda, our closest galactic neighbour, sits a measly 2.5 million light years away from Earth. Like the Milky Way, it's a spiral galaxy packed with stars. Some of those stars orbit one another. That's all good - but scientists have come to the realisation that a particular light source, one assumed to be binary star system in Andromeda, may actually be something far stranger. Like a pair of orbiting supermassive black holes a thousand times further away.