Nobody hass seen cold dark matter. Are ultra-cold (non-relativistic) neutrinos, below 1 fK (femtokelvin), an option for dark matter?
This is a question about normal neutrinos - electron neutrinos and muon neutrinos and tau neutrinos - and not any additional, invented neutrino types.
This is a question about neutrinos that are not in the cosmic neutrino background, which have a temperature of 1.95K.
The question is about cosmological dark matter, not about galactic dark matter.
Their density could be large enough to produce the observed cosmological dark matter density.
Due to their low temperature and low kinetic energy they would be undetectable.
They would be continuously emitted by the cosmological horizon in the same way as black hole radiation is continuously emitted by a black hole. Alternatively, they would arise automatically throughout the universe, whenever space grows in size.
Why is this not possible? Or why is it possible?
Given that the answers below imply that ultra-cold neutrinos cannot be dense enough because of their fermion character, could fermion condensation (via Cooper pairs, as in superconductors) solve the problem?