Cosmologists know that the well-understood parts of the universe—stars and other visible matter, as well as electromagnetic radiation and other familiar forms of energy—make up only about 5 percent of what’s really out there. The rest is dark matter and dark energy. The former is a substance that is all but invisible, save for its gravitational pull, and the latter is a mysterious force that powers the accelerating expansion of the universe. Dark matter and dark energy alike subtly influence the shapes and arrangements of galaxies and galaxy clusters, which collectively form filaments, sheets and voids in a vast “cosmic web.”