A wild-card in the debate over dark matter, the neutrino has one great advantage: Astronomers know they exist by the boatload. Roughly 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 a decided disadvantage: No one knows whether they have any mass at all. When they were first postulated by the Austrian physicist Wolfgang Pauli in the 1930s to explain the energy given off by radioactive beta 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.