There is more to the universe than meets the eye--a lot more. In recent years, scientists have discovered that 95 percent of the contents of the cosmos are invisible to our current methods of direct detection. Yet something is holding galaxies and galaxy clusters together, and something else is causing space to fly apart. Scientists call these invisible components dark matter and dark energy: "dark" because these phenomena do not emit light. Dark matter and dark energy are the most eagerly studied subjects in astronomy and particle physics today.