Sterile Neutrinos

Sterile neutrinos, hypothetical heavier cousins of known neutrinos interacting only via gravity, could explain dark matter at 7 keV masses and matter-antimatter asymmetry via leptogenesis. An anomalous X-ray line near 3.5 keV from galaxy clusters, detected by XMM-Newton and Chandra with a decay rate of 2.2 × 10⁻²⁷ s⁻¹, initially sparked hope, but Hitomi's higher-resolution instruments lowered the flux limit fivefold, casting doubt. Compared to axions (#3), sterile neutrinos tie more directly to neutrino physics, yet their gravitational-only interaction makes detection far harder than ADMX experiments. This candidate perches on the edge, awaiting future X-ray observatories like ATHENA to revive the elusive signal.