In the 1930s, astronomers made observations which tended to suggest that the orbits of stars in various galaxies did not have the expected relation between speed and distance from the galactic center. But it was not until 1975 that careful studies of a number of different galaxies, using new techniques, by astronomer Vera Rubin, showed conclusively that the speeds of stars in orbit in every galaxy tended to be independent of the distance from the center of the galaxy. If the visible mass of galaxies was responsible for star acceleration, then the speeds should vary as [1/r]^1/2, with distance. Astronomers and physicists quickly realized that almost all (90%) of the mass of galaxies resides in an invisible halo of unknown,individual particles sticking out into space for a distance around 10 times the visible galactic radius! Using an idea proposed by Einstein, “gravitational lensing,” astronomers have been able to map Dark Matter distributions all over the near universe in the past 30 years.