The neutrinos are not missing 'cosmologically'. Neutrinos are produced in the sun when two protons are fused into a helium atom.

p + p ---} 2H + neutrinoe

(I can't do a nu which is the greek symbol for a neutrino.) We think we know how much fuel the sun is burning, which means we should know the number of electron neutrinos being produced. We see far less than the predicted amount. One of two things is happening. Either we don't understand the process in the sun, and the number of neutrinos we see is really all that are being produced, or there is something funny going on with the neutrinos.

It is much harder to swallow the idea that what we know about the sun is baloney than it is to play with the properties of neutrinos. This is because neutrinos are notoriously difficult to measure and have a lot of uncertainty about their properties (like whether they have mass or not).

Now, it turns out that if they do have mass, one species (or flavour) of neutrino can turn into another flavour en route to Earth. This is called neutrino mixing. They can't mix if they have no mass. The amount to which they can mix is described by a thing called the Cabbibo angle, there is some good reason why it is an angle, but I'm buggered if I know what that reason is.

Reecent experiments at a Japanese detector (Super Kamiokande) indicate that there is definitely a difference in mass between types of neutrinos, indicating that they have some mass. Yay! If they have enough mass, the solar neutrino problem is solved. We don't know if they have enough mass yet. It's time to ask for more *chough*money*chough* and bigger detectors.

If they have enough mass, they could easily account for dark matter, and astronomers would be very happy. They would be hot dark matter.