No one knows for sure what makes up more than 80 percent of the matter in the universe. Though this so-called dark matter, which does not interact with light, has not been detected directly, scientists see evidence of its existence in gravitational interactions whose effects are visible in distant galaxy clusters. Now, using rapidly advancing techniques, physicists are mounting a major effort to detect the exotic particles thought to make up dark matter.
The advanced thin ionization calorimeter (ATIC), a balloon-borne instrument, is used to detect bursts of cosmic rays that may be evidence of annihilating collisions between dark-matter particles. ATIC is flown at McMurdo Station on Ross Island in Antarctica, where summer wind patterns carry the detector on a long flight around the South Pole. Here, workers prepare the balloon for takeoff at McMurdo in 2005.
Video: Watch an MIT physicist explain dark matter.
ATIC is tested before the 2007 launch.
Dark matter was first postulated by the Caltech astrophysicist Fritz Zwicky in papers published in the 1930s. Zwicky, shown here at the Palomar Observatory, calculated that there was more mass in a cluster of galaxies he was studying than could be accounted for by measurements from the telescopes of the day.
This image, a composite of images taken by the Hubble Space Telescope, shows how the pull of a ring of dark matter distorts the light from stars in distant galaxies.
The most promising technologies in the search for dark matter are systems for direct detection of WIMPs, or weakly interacting massive particles. The detector that physicists believe is likely to find WIMPs first, called CDMS (for “cryogenic dark-matter search”), is shown at bottom left. The two hexagonal boxes contain massive semiconducting crystals held at temperatures near absolute zero. When a WIMP strikes, the nuclei of the semiconducting material will recoil in a characteristic fashion, creating an electron hole and a small amount of heat that are detected by superconducting circuits and by a film of tungsten that acts as a very sensitive thermometer. Researchers hope to see about one particle per kilogram of crystal every three months.
Other WIMP detectors–including XENON, pictured here, and WARP (for “WIMP Argon Programme”), on the next page–are designed to capture nuclear recoil in vats of liquefied noble elements.
WARP, a WIMP detector designed to capture nuclear recoil in vats of liquefied noble elements.
The glass chamber in the center of the machine shown here is filled with a liquid that will evaporate, forming bubbles as large as a millimeter in diameter, when a WIMP strikes. Because heat or cosmic rays can also cause nuclear recoil, detection will need to be confirmed by multiple technologies.