The Search for Dark Matter; March 2003; Scientific American Magazine; by David B. Cline; 8 Page(s)

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.

For 70 years, astronomers have steadily gathered circumstantial evidence for the existence of dark matter, and nearly everyone accepts that it is real. But circumstantial evidence is unsatisfying. It cannot conclusively rule out alternatives, such as modified laws of physics [see "Does Dark Matter Really Exist?" by Mordehai Milgrom; Scientific American, August 2002]. Nor does it reveal much about the properties of the supposed material. Essentially, all we know is that dark matter clumps together, providing a gravitational anchor for galaxies and larger structures such as galaxy clusters. It almost certainly consists of a hitherto undiscovered type of elementary particle. Dark energy, despite its confusingly similar name, is a separate substance that entered the picture only in 1998. It is spread uniformly through space, exerts a negative pressure and causes the expansion of the universe to accelerate.