The concept of dark matter arose as a solution to a problem that has been puzzling astronomers for decades. Galaxies, when observed, rotate at a much faster rate than is expected for the estimated mass they contain. Astronomers and physicists have suggested that the extra mass may be accounted for by dim objects that our instruments cannot detect, or that the laws of gravitation are different over large distances. The most convincing argument, however, is that a previously unknown type of matter—dark matter—is clumped throughout the universe. We did not know about dark matter before now because it rarely interacts with regular (what physicists call baryonic) matter or with light other than through gravitation.