This diagram shows the ingredients of the universe. Astronomers have recently realized that the main ingredient is "dark energy," a mysterious form of energy that exists between galaxies and forces the universe to expand at an ever-increasing rate. The next-largest constituent is "dark matter," which is an unknown form of matter. The rest of the universe consists of ordinary matter, most of which is locked up in stars and clouds of gas. A tiny fraction of this matter is composed of heavier elements, the stuff of which humans and planets are made.