An image of the quasar 3C 186, taken by the Hubble Space TelescopeNASAs Goddard Space Flight CenterA rare collision between two supermassive black holes (SMBH) appears to have sent the resulting merger speeding through the universe, making it one of the fastest-moving black holes we have ever seen.Astronomers have long puzzled over how the gargantuan black holes at the centres of galaxies can grow to be so large. One possible route is for smaller but still extremely massive black holes to merge together, but there has been little direct evidence of this happening.