In the early 1970s Princeton University astrophysicist Jeremiah Ostriker was puzzling over telescope observations of distant galaxies. These spinning cosmic discs did not contain nearly enough stars and other visible material for gravity to hold them together. The answer, he realised, must be that a much larger mass of unobserved “dark matter” stopped them flying apart.
Though there had been scientific speculation about dark matter since the 1930s, Ostriker, who has died at the age of 87, played an instrumental role in convincing cosmologists that it really did pervade the universe. The consensus today is that dark matter has a total cosmic mass six times that of ordinary matter — close to the proportion calculated by Ostriker and his Princeton research group in a key 1974 paper. But astrophysicists still have little idea of what makes up dark matter.