MARINA DEL REY, Calif., Feb. 22 - Two years ago at a scientific meeting here, a team of physicists based at the University of Rome shook the world of physics by announcing that they had made what would have been one of the greatest discoveries in the history of science: a detection of particles that may flit through space almost unnoticed, in such numbers that they make up most of the mass of the universe.
Today, that claim came close to dying a quiet death as three separate groups, including one at Stanford University, said that despite their best efforts, they had been unable to find the so-called dark matter particles suggested by the Rome experiments.
"To tell you the truth, I don't think a lot of people believe the results," said Dr. Olivier Martineau of the Institute for Nuclear Physics in Lyon, France, who is working on a dark matter experiment called Edelweiss in the French-Italian Alps. He and his colleagues have seen no sign of the particles.
Still, many scientists here said that despite the apparent failure of the Italian experiment, the search for the particles was only now getting under way in earnest in half a dozen experiments around the world. Astronomical observations have shown that only a tiny fraction of all the matter in the universe is in bright stars or other visible matter.
The leading candidates for the invisible dark matter are difficult-to- detect particles swarming through space and called WIMP's, for weakly interacting massive particles, which all of the research groups are trying to find.
But if the WIMP's are out there, they seem to have eluded scientists so far. One after the other, scientists here said that while they had not yet achieved final proof that the work by the University of Rome physicists was wrong, no suggestion of anything resembling the startling results had turned up elsewhere.
Scientists here at the Fifth International Symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe also said that the Italian group, which conducted its work at the Gran Sasso National Laboratory east of Rome, had declined to address numerous questions about their methods that had arisen since the announcement in 2000.
Citing a health problem in the family of a researcher, Dr. Pierluigi Belli of the University of Rome, the Italian group canceled its presentation here and instead sent copies of the overhead projector slides Dr. Belli had planned to use in his talk. The slides reiterate the group's claim of having seen the signature of the dark matter particles in its detectors.
Dr. Rita Bernabei, the group's leader, said in an e-mail message that assertions that her team's findings were being overturned were incorrect and that criticisms of the work by physicists around the world had no basis in fact.
"Our analyses are so simple, standard and straightforward that every student in physics - without any prejudice - can easily understand them," Dr. Bernabei said.
The team declined to respond to many criticisms because "it is our policy to give no room to gossips," she added.
Those views were not endorsed at the dark matter conference, which is held every other year.
"Personally, I don't believe their results," said Dr. Richard Schnee, a physicist at Case Western Reserve University in Cleveland, "but I certainly wouldn't say that we have entirely ruled out their results."
Dr. Schnee works on the Cryogenic Dark Matter Search, an experiment that will move to a deeper site in an abandoned mine in Minnesota from its current base at Stanford. Sites deep underground screen many ordinary particles that can confuse the researchers.
Dr. Leszek Roszkowski, a particle physicist at Lancaster University in England, said that in an experiment with such profound implications, the Italian group's sensitivity to criticism was misplaced.
"It's an achievement that would go into history," Dr. Roszkowski said. "Anyone who makes any claims of this sort has to be prepared to be scrutinized."