Based on his measurements of the velocity of the stars, Zwicky calculated that the amount of mass required for the cluster to have sufficient gravitational pull was 400 times greater than the contribution of the measured luminous mass—the matter that emits light. To account for all that extra matter, Zwicky proposed the existence of what he named dunkle Materie, which is German for dark matter and sounds either more ominous or sillier depending on how you pronounce it.
