Now that neutrinos have been shown to have a rest mass, are they once again candidates for making up dark matter?
The nonzero rest mass of the neutrino was first proposed to explain why many fewer solar neutrinos than expected were detected in an experiment in the 1960s. The number of neutrinos reaching the Earth from the Sun can be predicted based on the knowledge we have of the solar fusion reactions that create the neutrinos. When predictions did not match observations, there were a few possible explanations. Either models of the Sun were incorrect, knowledge of neutrinos was inaccurate, or both. Most efforts focused on option two since there were several other observations of the Sun that supported the current solar models. To explain the solar neutrino problem, one theory is that neutrinos undergo oscillations. Simply put, neutrinos come in different flavors and the probability that a neutrino is a particular flavor can change as the particle propagates. If these oscillations do indeed occur (and there are experiments that believe to have detected them), then the neutrino would be required to have a rest mass.
So how does all this fit in with the mystery of dark matter? Neutrinos are one candidate for dark matter but only if they have a nonzero rest mass. Neutrinos interact only via the weak force and gravity which would explain we don't see dark matter can't be detected through interactions with light like baryonic (normal) matter. There are also so many neutrinos that even if they only had a mass one five thousandth that of the electron, the mass of all the neutrinos in the universe could make up for the missing matter. Neutrinos are the foremost candidate in the Hot Dark Matter theory which as explained here is only thought to be a possible explanation for dark matter in combination with Cold Dark Matter theory and not on its own.
This page was last updated June 27, 2015.