Ordinary Standard Model neutrinos _are_ dark matter particles, in a sense that they have mass, and they only weakly interact with everything else. IIRC they are not considered to be a satisfactory candidate because nearly all neutrinos in the Universe are relativistic, whereas we know that observed dark matter is "clumpy" and thus must be composed of non-relativistic particles. Why we think that neutrinos are relativistic? (1) all neutrino-generating processes today are generating relativistic neutrinos, and (2) vast majority of neutrinos should be primordial, from ~1s after Big Bang, when neutrinos decoupled. They must have temperature of ~2K now, which is ~0.2meV, but with their low masses they must have (nearly) relativistic velocities even today. However, argument (2) assumes that we do not miss anything and that there are no unknown or unanticipated process which was generating "cold" neutrinos. Let's for the sake of argument assume that there was some such process, and there is a subset of CvB neutrinos which is actually much colder than 2K. Is this possibility ruled out? IIRC low-energy neutrinos have low cross-sections, and CvB neutrinos were never directly observed (neither their density nor their velocity). Do I remember this wrong?