Dark matter and dark energy have little effect on conventional matter over familiar distances. Instead, they make their presence known through their prodigious gravitational effects. In tracking them down, therefore, astronomers have had to study gigantic assemblages of matter, extending across spans of millions and billions of light-years. Perhaps the first to take that sweeping viewpoint was the Swiss-American astronomer Fritz Zwicky. In the 1930's, Zwicky traced the motions of individual galaxies within great clusters of galaxies and made a remarkable discovery: the individual galaxies are moving too fast to be held together in a cluster by the force of gravity exerted by the starry matter visible within them. From his measurements, Zwicky concluded that the great clusters of galaxies must be held together by the gravitational effect of some unseen mass, which he dubbed "dark matter."