Neutrinos are particles that can travel through 1 light year of lead without interacting with any of the lead atoms. It is extremely small and light and is responsible for a small portion of dark matter. Professor Lahav believes neutrinos are a minor component of cold dark matter, the mysterious "stuff" which comprises some 25% of the Universe and more than 80% of matter in the Universe.
"The neutrino is squeezed into that slice [of the Universe] that is dark matter. But it probably accounts for less than one percent of that dark matter," he told BBC News.
The neutrino particle comes in three "flavours": muon, tau and electron. In a recent experiment, physicists caught a neutrino in the act of changing from one type to another.
BBC News - Neutrino 'ghost particle' sized up by astronomers