It has been suggested several places that dark matter consists of cold neutrinos. Here are some numbers regarding this notion. Some estimates are that the rest mass of an electron neutrino is about .1 ev or 10−13 ergs or 10−34 g. There are good reasons to think that the mass is not much more than this.

Since neutrinos interact little with other matter they do not thermalize quickly and thus come to a common temperature with their surroundings. Modern sources of neutrinos, such as super novae, produce hot neutrinos and this source is not sufficient. We will ignore those here.

A particle in thermal equilibrium with the cosmic microwave background radiation of 2.7 K has an energy of 2.7 K • 8.617×10−5 eV/K = 2.3 ×10−4 eV = 3.7×10−16 erg. A thermalized neutrino would have a velocity of 2×109 cm/s. The rotation velocity of a galaxy is about 1.5×107 cm/s. This seems to say that even at 2.7 K, neutrinos are much too hot to stay near any galaxy to serve as its dark matter. They seem to have many times escape velocity. This calculation does not account for the source of the 1090 neutrinos necessary to play the role of dark matter.

Postulating primordially cold neutrinos seems perhaps about as exotic as other proposals such as the supersymmetric candidates. One messes with thermodynamics with even more caution than fundamental particle physics; it has been around longer, and making better predictions. Still I can sort of imagine an exotic supply of cold neutrinos that were there from the beginning, even before inflation. They did not thermalize even then. As dense as it was back then, it did not last long. Anything besides neutrinos was pretty much thermalized. John Denker expands on this note.


What binds dark matter to galaxies? Gravity is not a good enough answer; there is the capture question. In celestial mechanics it takes three bodies to bring an elliptical orbit into existence. Matter clumps because collisions between clumps are inelastic and produce heat which turns hyperbolic orbits into elliptical orbits. Further collisions consolidate masses limited only by conservation of angular momentum which cannot be turned into heat as can translational kinetic energy. All of this seems due to the Pauli exclusion principle which seems not to apply to dark matter. Perhaps dark matter is not only primordially cold but also pre-captured at the time of the quantum density fluctuations which account for the galaxies according to some theories.

Scientific American says that Andrew Gould says that the solar system should have ejected dark matter the same way in cleared out most of the primordial debris.

Some Things its not

Its not tiny red dwarf stars.
Its not even baryonic.
On the other hand it may merely be ‘nomad planets’. Cold (1.9 K) but not cold enough. more