Dark matter
Dark matter was first proposed by Jan Oort in 1932 as a reason for the spinning speeds of stars in the Milky Way. Fritz Zwicky in 1933 used dark matter to explain "missing mass" in the spinning speeds of galaxies in clusters. Later, many other observations have suggested that there is dark matter in the universe. The spinning speeds of galaxies, gravitational lensing of background objects, the temperature distribution of hot gas in galaxies and clusters of galaxies: these are some of the examples that make scientists believe in dark matter.