Besides the energy released as photons (visible light and other electromagnetic radiation) from a fusion reaction, there are also many neutrinos. When these are measured on earth from fusion and collider experiments, the measurements fall within theoretical bounds showing theory to be correct. Neutrino flux from the sun, which uses the same reactions as have been tested here on earth, has been measured to be one to two thirds what it should be. This is a significant difference from theory, and has caused a lot of confusion among physicists -- and science fiction writers :-) -- since it was noticed in 1967.

Neutrinos come in three flavors, electron, muon, and tau. They are differentiated by the particle that is generated when they interact with other matter (such as an atomic nucleus) -- they release either electrons, muons or tau particles. Electron and muon neutrinos can be measured with instruments like the Super-Kamiokande and the Sudbury Neutrino Observatory, whereas tau neutrinos can only be found through the use of high-density emulsions to capture the released tau.

From observations made by the Super-Kamiokande, the reason for the missing neutrinos can only be neutrino oscillation. Super-Kamiokande measures this by taking readings on both sides of the earth, and comparing the difference. Since a negligible amount of neutrinos are absorbed by the earth, the difference describes the percentage of neutrinos that have oscillated into an undetectable form. Thus, the sun releases neutrinos in the proportion it should, but some of them oscillate into undetectable tau neutrinos before they reach earth, throwing off our measurements.

Besides being interesting because they explain the deficit of solar neutrinos, verified neutrino oscillations also mean verified neutrino mass. When suggested by Wolfgang Pauli in 1930 as an explanation for missing energy in beta decay, neutrinos were thought to have zero mass. Now, however, we see that neutrinos oscillate, and thus must have some mass due to quantum wave theory. Since neutrinos vastly outnumber every other kind of particle in the universe, their mass will be a determining factor in whether or not the universe eventually undergoes a Big Crunch. In fact, it has been shown that if neutrino mass was over 100 eV (where electrons mass something like 511,000 eV), the universe would've already collapsed. Also, neutrino mass is important because it pokes holes in the current electroweak theory of particle interaction, pointing to need of a better way of conceptualizing physics.