The cosmos is mostly made of something we cannot see. That was the conclusion astronomers started to reach in the 1930s by looking at galaxy clusters, which should have blown apart unless some “dark matter” was binding them together. Scientists started taking the idea more seriously in the 1970s, when astronomers studying how fast galaxies rotated found the same thing. Soon researchers realized that dark matter was unlikely to be made up of normal matter and radiation. By now it seems nearly inescapable that more than 90 percent of the stuff in the universe that clumps together under gravity is some exotic material, perhaps a new particle left over from the big bang.