Until the past decade or so, two mysteries bedeviled physicists. One was the attempt to catch neutrinos. These elusive particles are so light, less than 1/7 the mass of an electron, they pass through everything virtually unhindered. They are passing through you, the Earth, everything in the Universe at all times.
Until 1998, we thought neutrinos had zero mass, but the massive underground detectors built to capture them by then showed they did have that slight mass mentioned above. The detectors also proved they were affected by the weak force, the force that causes radioactive decay and permits quarks to morph into antiparticles, among other phenomena. One of these detectors is shown below. It is the Sudbury Neutrino Observatory (SNO) in Sudbury, England. Two thousand meters below the ground, this 12-meter sphere is filled with heavy water and surrounded with light detectors.
Up to that time, the standard model predicted three types of neutrinos—the Muon, Tau, and electron neutrinos. That weak force can cause the various types to switch from one to the other as they course through the Universe.
Since 1998, astronomers and astrophysicists have found strong evidence of a fourth neutrino. Theorists had insisted that, since the three known types did have mass, however small, there had to be a fourth type, heavier than the other three. The orbiting observatory Wilkinson Microwave Anisotropy Probe (WMAP), since 2001, has been mapping tiny fluctuations in the radiation left over from the Big Bang. In data just released last January, there is evidence that a fourth neutrino existed in the early Universe.