Galaxies shine light on dark matter.

More than 80 years ago, Albert Einstein made an astounding assertion: Gravity bends light. A clump of matter can act like an irregularly shaped piece of glass, altering the path of light rays from an object that lies behind it and creating a distorted image.

The material doing the distorting, an effect known as gravitational lensing, needn't be visible stars or galaxies. The unseen material known as dark matter, which astronomers believe pervades space and weighs 10 times as much as all the visible stuff, should also bend light (SN: 1/8/00, p. 30).

Four teams of astronomers have now independently found signs of lensing due to dark matter, providing fresh evidence for the existence and distribution of this massive but unseen component of the cosmos.

Gathering the evidence required years of study. Dark matter typically elongates the image of a perfectly round galaxy by a tiny amount. Because no galaxy is exactly round, however, images of a single object can't reveal whether it's been distorted. Images of many galaxies lying behind a single dark-matter clump will all suffer elongation along the same direction, allowing astronomers to tease out the lensing.

David M. Wittman of Bell Laboratories in Murray Hill, N.J., and his colleagues examined 145,000 distant galaxies at the Cerro Tololo Inter-American Observatory in La Serena, Chile. Although the galaxies reside in patches of sky that contain no visible foreground objects, the team discerned the effect of gravitational lensing. The lensing must be due to dark matter, the researchers conclude in the May 11 NATURE.

Three other groups have posted similar findings on the Internet. Their reports are numbers 0003338, 0003008, and 0002500 at http://xxx.lanl.gov/abs/astro-ph.

These studies "could be the first shots of a revolution in our ability to measure dark matter," comments Max Tegmark of the University of Pennsylvania in Philadelphia in the same issue of NATURE. Ultimately, he says, such observations will produce dark-matter maps of the entire sky.