From Tom Ritchey
You ascribe the origin of dark matter to Vera Rubin, an astronomer at the Carnegie Institution, and say she “spotted this anomaly in the late 1970s” (19 March, p 33). Maybe she did, but Caltech professor Fritz Zwicky applied the “virial theorem” that relates potential and kinetic energy to the Coma cluster of galaxies in the early 1930s, obtained evidence of unseen mass, and started the debate on what we now call dark matter.
Bromma, Sweden
