born in Bulgaria, who worked in America. In 1933 he found that the mass–luminosity ratio for clusters of galaxies was fifty times that for lone galaxies—an early indication of dark matter, which he attributed to intergalactic matter and dwarf galaxies. In 1934 he and W. Baade coined the term ‘supernova’ to distinguish such enormous eruptions from the less powerful novae, and proposed that a supernova explosion leaves behind a neutron star, confirmed when the Crab Pulsar was identified in 1968. Zwicky observed supernovae in other galaxies, and began a long study of galaxy clusters (see Zwicky Catalogue). He also drew attention to intergalactic matter, in particular to bridges of stars between neighbouring galaxies.