What Is Dark Matter?

Gabriella Sciolla, an associate professor of physics at MIT, hopes to validate supersymmetry through experiments on dark matter. Physicists know, from observing the gravitational interactions of galaxies and other celestial objects, that there is much more mass in the universe than they can account for by looking for the kinds identified by the standard model. This missing mass is called "dark matter" because it doesn't interact with photons. It cannot be seen with optical or x-ray telescopes. "For sure, I am a little biased, but for me, the most interesting open question in physics is, What is dark matter?" says Sciolla. One simple explanation is that it is made up of one or more of the supersymmetric particles.

In the bowels of Building NW13, in a windowless cinder-block room that her research group calls "the dungeon," Sciolla is testing a new apparatus called the Dark Matter Time Projection Chamber--essentially a large stainless-steel tank of gas flanked by two digital cameras. The principle behind the detector is simple. When a particle of dark matter strikes a gas atom, the atom will recoil, knocking loose electrons that will be detected by the cameras. By tracing the paths of these electrons, Sciolla will be able to see not only that a particle struck, but from which direction. That will be important in establishing that the detector is actually seeing dark matter, not something else. If, as many physicists believe, our galaxy is rotating through a stationary region of dark matter, then the dark matter should strike the atoms in Sciolla's detector like rain hitting the windshield of a moving car. The direction of this "rain" should vary by about 90º every 12 hours, because the axis of Earth's rotation is about 45º with respect to the dark matter.

Sciolla and her research group will station their detector in an underground lab to isolate it from cosmic rays, a major source of noise, and they'll spend 2009 gathering preliminary data to prove that the concept works. In a year, Sciolla hopes to have a one-cubic-meter detector that will be 50 times as sensitive; in five years, she hopes to have a detector as large as a few hundred cubic meters.

Finding dark-matter particles would be the physicist's equivalent of hitting the jackpot. "All these big questions in physics are somehow connected," Sciolla says. "Dark matter is the one answer that would satisfy so many different unanswered questions in different fields of physics." Detecting it would provide strong evidence for supersymmetry.

If dark matter turns out to consist not of supersymmetric particles but of axions, hypothetical particles that Wilczek did important work to describe, that finding could get at another huge question. Axions figure prominently in an esoteric theory that explains why matter--as opposed to antimatter--prevails in the universe, even though the Big Bang produced all the particles and their antiparticles in equal numbers.