"Six thousand billion of them are going through your body every second." That's physicist Lawrence Krauss on neutrinos, on the June 15th weekly Science Talk podcast. "Neutrinos are the lightest elementary particles we know of. The name comes from the fact that they had to be neutral because we couldn't see them in detectors. But they had to be light. So Enrico Fermi called them "a little neutron," in Italian is neutrino. So they were baby neutrons, which were the only other neutral particles at the time that were known."

Now astrophysicists have put a best-guess upper limit on the mass of the neutrino. The research is being reported this week at the Weizmann U.K. conference at University College London and will appear in an upcoming issue of Physical Review Letters.

The work took advantage of the huge 3-D galaxy map called Mega Z. Because there are so many neutrinos, their tiny masses add up to influence the overall distribution of galaxies. And the researchers say the galactic map depends on a neutrino's mass being no more than 0.28 electron volts, less than a billionth of the mass of an atom of hydrogen, the lightest element. "These are the most interesting particles in nature."