Sterile Neutrinos

Sterile Neutrinos are a type of neutrino even more elusive than regular neutrinos. If found, they could be a good candidate for dark matter as well as an addition to the standard model. They also might explain 

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The weak force has a unique property in that it cares about the spin of particles. Specifically, the weak force only interacts with left-handed particles (go to the page on particle properties in section 1 for more information). The weak force can also interact with right-handed antineutrinos.

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This means that right-handed neutrinos and left-handed antineutrinos do not interact with the weak force. For neutrinos, which normally only interact with the weak force and the gravitational force, this means that they are even more elusive. Now, those neutrinos would only interact with the gravitational force, making detection nearly impossible. These two types of neutrinos are called sterile neutrinos.

Experiments such as the one at MiniBooNE might support the existence of these neutrinos. However, these results are in conflict with others, such as the ones at the IceCube observatory and observed by the Planck Satelite. More experimenting will need to be done.

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If it does exist,  the sterile neutrino would have a relatively low mass of around 1eV, heavier than regular neutrinos, but maybe not a heavy enough candidate for dark matter.