# Could a 4th generation neutrino be a viable candidate for dark matter?

Dark matter does not interact with electromagnetic radiation. Neutrinos also don't interact via electromagnetism, which would make neutrinos a good candidate for dark matter. However because of the extremely low mass of the known three types of neutrinos they can only supply a tiny fraction of the dark matter in the universe (see the section of the Wikipedia page comparing baryonic and non-baryonic matter).

But I have heard there have been speculations in the past, that there could be a fourth type of neutrino, which should be much more heavy that the other three types of neutrinos. Wouldn't such a heavy 4th generation neutrino be the perfect, natural candidate for dark matter?

Weak physics rates (I think the $Z^0$ lifetime is generally quoted as giving the strongest single constraint) give pretty strong reason to doubt the existence of the fourth generation of 'normal' neutrinos with a mass lower than $\approx 45\,\mathrm{GeV}$, but so-called "sterile neutrinos" (which don't couple directly to the weak interaction, but do mix with the better know flavors) have been on the radar as candidates for some time.