LOS ANGELES – For the first time, physicists have confirmed that certain subatomic particles have mass and that they could account for a large proportion of matter in the universe, the so-called dark matter that astrophysicists know is there but that cannot be observed by conventional means.

The finding concerns the behavior of neutrinos, ghost-like particles that travel at the speed of light. In the new experiment, physicists captured a muon neutrino in the process of transforming into a tau neutrino.

Researchers had strongly believed that such transformations occur because they have been able to observe the disappearance of muon neutrinos in a variety of experiments.

But the research announced Monday marks the first time that the appearance of a tau neutrino has been directly observed. Physicists from CERN — the European Organization for Nuclear Research — in Geneva and the Italian National Institute of Nuclear Physics’ Gran Sasso National Laboratory were involved.

“This is an important step for neutrino physics,” CERN Director-General Rolf Heuer said. “We’re all looking forward to unveiling the new physics this result presages.”

Astrophysicists have inferred the existence of dark matter because the total amount of visible matter is insufficient to account for gravitational effects. It is estimated that dark matter accounts for 80 percent of the mass of the universe and visible matter only 20 percent.

In theories used to explain the behavior of fundamental particles, called the Standard Model, neutrinos have no mass. But if they have no mass, they cannot oscillate between muon and tau forms. The fact that they do oscillate indicates that they have mass and that the Standard Model needs reworking.