Walking to the back of the warehouse, past piles of equipment, Nelson pointed at the skeleton of an old experiment, beyond the point where the tree-trunk-sized pipe of the historic accelerator cut off. It's there, he said, that a soon-to-be-constructed experiment will see?"or quickly rule out?"one of the most popular new candidates for dark matter. Almost a century ago, the Swiss astrophysicist Fritz Zwicky described a galaxy cluster that appeared to rotate too fast to be held together by its visible mass. He proposed that invisible matter was lending its gravity to the situation. The evidence has grown and grown, and researchers now believe that 85 percent of the universe's matter is hidden. But the mystery of dark matter's identity has endured.