It has long been known from the speeds with which stars, gas, and dust orbit about the centers of spiral galaxies that as much as 90% of the material in these galaxies is in the form of a "dark matter halo." Since the dark matter does not emit light, it is extremely difficult for astronomers to know just how far it extends beyond the visible region of a galaxy, making it impossible to know the total mass of the galaxy with any certainty.

Recently, however, Brainerd and her colleagues (Professor Roger Blandford at Caltech and Dr. Ian Smail at the University of Durham) used gravitational lensing, a technique based on Einstein's theory of General Relativity, to directly measure the extent of dark matter halos. Their results suggest that for typical spiral galaxies the dark matter extends to distances on the order 2 to 4 times the visible radius of the galaxy, resulting in a galaxy mass that is equivalent to one thousand billion (1,000,000,000,000) times the mass of the sun.

The total amount of dark matter in the universe may, in fact, determine its fate - either eternal expansion or an eventual halt. "However, in order for there to be a large effect on the fate of the universe, significantly more dark matter is necessary than that which exists in galaxies," says Brainerd. "What's really important that we've developed a valuable tool that can be applied to all galaxies and works at larger distances than previous methods. It allows us to obtain very basic knowledge about the structural parameters of galaxies - the luminous "bricks" out of which the universe is assembled and that define the visible structure of the universe."

Photo: Hubble Space Telescope image of the galaxy cluster Abell 2218. The entire cluster of galaxies acts as a gravitational lens for all galaxies that are more distant than the cluster, resulting in a coherent pattern of distorted images seen through the cluster. The thin "arclets," which appear as fragments of concentric circles centered on two primary concentrations of mass within the cluster, are the highly distorted images of galaxies that are located at distances much greater than that of the galaxy cluster. By mapping the distortion pattern due to objects such as Abell 2218, astronomers have been able to make accurate maps of the distribution of mass within clusters of galaxies.