Within the confines of the ordinary, vision is the most reliable tool we have. But some of the most extraordinary parts of nature, those that lie at the frontiers of science, can't be seen at all. Dark matter, for example, the invisible, mysterious material that makes up 22 percent of the stuff in the universe, is one of the great scientific unknowns, a substance nearly six times as abundant as ordinary matter but made up of fundamental particles we haven't yet identified.