The quasar OJ287 contains two black holes (this slightly dated illustration lists the larger black hole's mass as 17 billion Suns, though researchers now estimate it is 18 billion Suns). The smaller black hole crashes through a disc of material around the larger one twice every orbit, creating bright outbursts (Illustration: VISPA) The most massive known black hole in the universe has been discovered, weighing in with the mass of 18 billion Suns. Observing the orbit of a smaller black hole around this monster has allowed astronomers to test Einstein's theory of general relativity with stronger gravitational fields than ever...