A proposed experiment that would make ancient astronomers proud could end speculation about dark matter.
Scientists have long known that most stars orbit their galaxies so rapidly that gravity wouldn't be strong enough to keep them from flying away. The mainstream explanation is that some yet-undetected, or dark, matter adds mass to galaxies, increasing their gravitational tug.
Last year, astronomers reported the most dramatic evidence yet for dark matter (SN 8/26/06, p. 131: Enlightened: Dark matter spotted after cosmic crash). But a few scientists remain skeptical. In an alternative theory, called modified Newtonian dynamics, or MOND, forces such as gravity would produce small accelerations in addition to that which standard Newtonian physics predicts. MOND would explain the orbits of stars without any need for dark matter.