OK, here we go:
First, E.G.'s take (which has been sitting in my inbox for a few hours now, sorry):
Everyone knows about electrons --- negatively charged atomic particles that orbit nuclei. Electrons are light, about 1000 times lighter than protons or neutrons. They are also stable, which means they last forever. There are two heavier cousins of the electron, the mu and the tau leptons, which are produced at accelerators and decay in a fraction of a second.
Correspondingly, there are electron, muon, and tauon neutrinos. Each flavor is distinguished by how it is created or destroyed --- the interaction of an electron neutrino can turn it into an electron, and so on. Unless something special happens, a neutrino of a given flavor would stay the same flavor as it travels along. However, neutrino experiments have shown that, for example, a beam of electron neutrinos can turn into a mixed beam of electron and muon neutrinos. This can only be understood with quantum mechanics.
This means that a neutrino of a given flavor does not have a definite mass. Rather, it is a superposition of a neutrino with definite mass m1 and another with definite mass m2 (and thus neither state has a definite flavor). Imagine a stopwatch with two second hands, one which runs slightly faster than the other, say 59 seconds to go around. When both hands are aligned, we have an electron neutrino. As the hands progress, one will fall farther and farther behind the other until it points in exactly the opposite direction of the other. At that instant, we have a muon neutrino. And so on.
How fast the hands go around the face depends on the neutrino masses, but the only thing we can measure is the relative directions of the two hands, which depends on the difference of the neutrino masses. If the neutrino masses were equal, then there would be no observable effects and the original electron neutrino would remain one. From other observations, we know that the neutrino masses are about one billion times smaller than the proton mass. Neutrino oscillation experiments tell us that the differences of masses are even smaller.
Dark matter being Kaluza-Klein excitations due to extra dimensions, and
observed by annihilations into neutrinos. Could be. Who knows?
Uh. I'll work on him, and see about translating this a bit further.
Meanwhile, dark matter ain't necessarily dark -- the "dark" has more to do with "unknown" than "lack of light". Neutrinos may well be dark matter ("dark matter" is basically just "the matter that must exist according to the Standard Model but we can't seem to find anywhere"), and they are everyplace, much like light but even more impervious to obstacles. They zip through most anything.