Neutrinos

• have no charge, and little, if any, mass
• almost never interact with other particles
• pass right through earth without interacting
• 1/sec pass through you...and only one will interact in your lifetime
• produced only in weak interaction decays
• observation of radioactive decay of neutron reveals the neutrino's existence.

sum of momenta of electron and proton
       equal to the original momentum of the neutron
some other particle accounts
       for the missing momentum: the neutrino neutrinos: produced by weak interactions in stars and supernova
       99% of energy
       a lot of them in the Universe...the missing mass?
       like 3 degree big bang photons