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.