The dark side of the universe

One of the greatest mysteries in the cosmos is that it is mostly dark. Not only is the observed night-sky dark, but so is most of the matter in the universe. For every atom currently visible in planets, stars and galaxies, there exists at least five or six times as much "dark matter" in the universe. Astronomers are seeking to unravel the nature of this mysterious but pervasive dark matter, and determine whether it can be detected. There is also a dark force, dubbed "dark energy" and originally postulated by Einstein in the form of the cosmological constant, that is systematically accelerating the universe and accounts for two-thirds of its mass-energy density. Understanding the nature of dark matter and dark energy present two of the greatest challenges in physics.