In the 1920s, astrophysicist Fritz Zwicky discovered that the galaxies of the Coma supercluster, 300 million light-years away, were traveling around the cluster’s gravitational center at speeds much too fast to keep them from flying away. He realized that some sort of matter, unseen by scientific detectors, might be hiding within the cluster. He called it “dark matter.”

In 1976, another astrophysicist, Vera Rubin, found that outlying stars orbiting galaxies also traveled too fast for the amount of visible mass within those galaxies. Thus, the dark matter problem exists not only within large groups of galaxies but inside the galaxies themselves. Overall, there’s about six times as much dark matter as visible matter in the universe.

None of the usual suspects—non-luminous clouds, rogue planets, even black holes—can come close to accounting for the excess matter. If dark matter participated in nuclear fusion, there’d be much more helium in the cosmos because, during the first few minutes of the universe, hydrogen was converted into helium so quickly that 10% of all atoms became helium. This isn’t nearly enough to account for all the mass in the universe, including dark matter. Dark matter exerts gravity but otherwise doesn’t interact in any detectable way with the mass around it.