Illustration of the binary black hole found in the centre of the nearest quasar to Earth, Markarian 231. Quasars are extremely luminous centres of galaxies. This pair of black holes orbit each other generating tremendous amounts of energy that makes the core of the host galaxy outshine the glow of its population of billions of stars. Observations of the ultraviolet light by the NASA/ESA Hubble Space telescope has shown, surprisingly, light diminishing close to the central black hole. Astronomers deduced that a smaller companion black hole has cleared out a donut hole in the accretion disk, and the smaller black hole has its own mini-disk with an ultraviolet glow.