Another intriguing dark matter candidate is a sterile neutrino. Whereas the three known neutrino species are far too light to constitute dark matter, it is possible that one or more additional neutrino types, that do not interact via the fundamental interactions of the standard model of particle physics could make up the dark matter. These sterile neutrinos could, however, mix with ordinary neutrinos.
In the past few years, several X-ray astronomy groups [123, 124] have found evidence for a 3.5 keV line in clusters of galaxies and in M31. This line would be consistent with a dark matter origin, corresponding to a 7 keV rest mass sterile neutrino with vacuum mixing with active neutrinos sin2 2θ ∼ (2 − 20) × 10−11. Fig. 15 illustrates some of the observations. However, others argue against this interpretation, e.g. Ref. 125 claims that the line is not seen from the dwarf galaxy Draco and thus the 7 keV sterile neutrino is ruled out. This is a subject of deep controversy.
Figure 15. Sterile Neutrinos: Observations consistent with and bounding the sterile neutrino mass and mixing angle to ordinary neutrinos (figure courtesy of K. Abazajian “Cosmology of Sterile Neutrinos,” in preparation (2016)).
Theoretical studies of sterile neutrinos are also ongoing. The sterile neutrino is a singlet under the standard model; it is likely a right handed neutrino. The production of these particles is difficult. If thermal, they tend to overclose the universe. Other mechanisms [126, 127, 128] or resonance using a large lepton asymmetry  are difficult but being investigated. In many models the sterile neutrino constitutes warm dark matter, which leads to testable predictions such as the core/cusp of galaxies and the numbers of substructures (objects smaller than our Galaxy), see e.g. Ref. 130.