The universe around us is not what it appears to be. The stars make up less
than 1 percent of its mass; all the loose gas and other forms of ordinary matter, less than 5 percent. The motions of this visible material reveal that it is mere flotsam on an unseen sea of unknown material. We know little about that sea. The terms we use to describe its components, "dark matter" and "dark energy," serve mainly as expressions of our ignorance.
This article was originally published with the title The Search for Dark Matter.