A mile beneath the Black Hills of South Dakota, in a former gold mine now devoted to one of science's most patient pursuits, researchers have recorded a faint but tantalizing signal that may represent the first direct encounter with dark matter — the invisible substance comprising 85 percent of all matter in the universe. Scientists at the Sanford Underground Research Facility observed a particle interaction consistent with a weakly interacting massive particle, or WIMP, though they have chosen their words with deliberate caution, speaking of hints rather than certainty. The moment belongs to