rt for Weakly Interacting Massive Particles, the WIMPs are predicted by theory but have so far not been found. With weird names like photino and masses of perhaps 10 to 100 times that of the proton, WIMPs could account for lots of dark matter if, as some theories are supposed to predict, they are common in the universe. NeutrinosThe apparent “wild-card” in the debate over dark matter, the neutrino has one great advantage: Astronomers know they exist by the truckload. Supposedly one billion neutrinos exist in the universe for every proton or electron, so they could add a huge amount of mass to the dark matter total. But the neutrino also has ahuge bust: No one knows whether they have any mass at all. When they were first found by some Austrian physicist in the 1930s to explain the energy given off by some sort of radioactive decay, neutrinos were thought to have no mass and to travel at the speed of light. Some experiments, however, seem to indicate that the neutrino has a very small mass (millions of times less than a proton) and moves at close to the speed of light. If neutrinos have even the slightest mass, they are so numerous they could make up a significant fraction of the dark matter.RadioactivityOne of the “key” moments in our noble scientists’ quest to comprehend the structure of matter came when they realized that not all elements are stable. The nuclei of many heavy elements, such as uranium, radium, and plutonium, are unstable, spontaneously goes all crazy and decays into other nuclei and releasing energy in the process. This radioactivity can occur in any of three ways: alpha decay, beta decay, and gamma decay. In the first, an alpha particle (the nucleus of a helium atom, which consists of two protons and two neutrons) comes shooting out of the nucleus at high speed. In the second, an energetic beta particle (an electron or its antiparticle, a positron) is emitted. And in the third, which usu...
