“They have not found dark matter,” he said. “There is nothing smacking you in the face to make you think there is something there.” But as the sensitivity of the detector increases, he added, “If there is anything in there, it should become apparent.”

The announcement at the Homestake site capped a morning of ceremony, which included Gov. Dennis Daugaard of South Dakota and members of the State Legislature, at what amounted to a coming-out party for LUX and for the Sanford Underground Research Facility, a lab being developed in the old mine with a mix of state and private money, as well as support from the Energy Department. The lab is named after the philanthropist T. Denny Sanford, who donated $70 million to get it going.

LUX is the latest in a long series of ever-larger experiments that have occupied and taunted the world’s physicists over the last few years. They are all in abandoned mines or other underground places to shield them from cosmic rays, which could cause false alarms. Daniel McKinsey, an associate professor of physics at Yale and a spokesman for the LUX group, said in an interview that the biggest source of noise in the LUX device was trace radioactivity in the detector itself.

Larger instruments are already on the drawing boards of LUX and other collaborations, but physicists say the experiments are already sensitive enough to test some versions of dark matter that have been proposed, including the idea that dark particles interact with ordinary matter by exchanging the recently discovered Higgs boson. Dr. Weiner said he held his breath every time new results from a dark matter experiment were released.

Dark matter has teased and tantalized physicists since the 1970s, when it was demonstrated that some invisible material must be providing the gravitational glue to hold galaxies together. Determining what it is would provide insight into particles and forces not described by the Standard Model that now rules physics, not to mention a slew of Nobel Prizes.

Physicists’ best guess is that this dark matter consists of clouds of exotic subatomic particles left over from the Big Bang and known generically as WIMPs, for weakly interacting massive particles, which would weigh several hundred times as much as a proton but could nevertheless pass through the Earth like smoke through a screen door. They are a generic feature of a much-hyped idea known as supersymmetry.

Particle physicists have been hoping to produce these particles or other evidence of supersymmetry in the Large Hadron Collider outside Geneva or to read their signature in cosmic rays from outer space. No one has ever claimed to have seen such a heavy WIMP, in space or underground, but another experiment in another mine, the Cryogenic Dark Matter Search, claims to have recorded three events that could have been low-mass dark matter particles, only a few times heavier than a proton.