Could not slow neutrinos orbit galaxies and clusters, thus comprising a large component of even COLD dark matter? Cold dark matter is the type of dark matter that is most eminently neutrino-free. But neutrinos themselves suffer a large survivorship detection bias (“all the neutrinos you can detect necessarily have to have relativistic speeds”) https://physics.stackexchange.com/questions/267035/where-are-all-the-slow-neutrinos
By the same principle that there are more pebbles than boulders, and more red dwarfs than blue giants, slow neutrinos should be more abundant than the fast, detectable ones.
Non-relativistic (slow) neutrinos - in particular those going at below the galactic escape velocity - could be a major competent of cold dark matter: slow neutrinos could be slow enough to orbit galaxies and galactic clusters and thus form a significant amount of cdm, which should be shaped as a galactic halo.
If neutrinos are produced at all speeds, those stellar (modern) neutrinos below galactic escape velocity will accumulate indefinitely. This should generate a halo-shaped cloud, and this could comprise a large part (of course 15% are MACHOs etc https://ned.ipac.caltech.edu/level5/Sept17/Freese/Freese4.html - I am assuming here a multi-component DM) of cold dark matter.
These neutrinos would have to be really cold. The cosmic neutrino background is at 1.9K, and they are considered hot dark matter, because they would have been highly relativistic at the epoch of structure formation. To be considered cold dark matter, and also to be captured in orbits in galaxies, the neutrinos would have to be much colder - totally non-relativistic now.
Let's assume an average energy then of about 0.1 eV for each neutrino (similar to their likely rest mass energy). In order to account for $\Omega_{\rm CDM}\sim 0.3$ there would need to be $5\times 10^{10}$ per cubic metre, or about $10^{10}$ per cubic metre per flavour, on average over the universe.
To account for dark matter in galaxies (e.g. the Milky Way) we need $\sim 10^{12} M_\odot$ within about 100 kpc, meaning a number density of neutrinos of $10^{14}$ per cubic metre.
These neutrinos are spin 1/2 fermions and thus would have a Fermi energy of about $5\times 10^{-3}$ eV. That would mean if they were any colder than 6 K, they would be degenerate and exert a degeneracy pressure. This would be sufficient to prevent the halos forming - Tremaine &amp; Gunn (1979) showed that cold dark matter halos cannot be made from low mass leptons like standard neutrinos.
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A back of the envelope Tremaine-Gunn limit (see also Boyarsky et al. 2009) is to assume that the escape speed of the galaxy is $v$, it's dark matter halos has radius $r$, the total mass is $M$ and the neutrino mass is $m$.
The number of available quantum states to spin half fermions in this volume, up to a momentum $mv$ is
$$ N = \left(\frac{4\pi r^3}{3}\right) \left(\frac{8\pi}{3}\right) \left(\frac{mv}{h}\right)^3$$
We can write
$$ v = \left(\frac{2GM}{r}\right)^{1/2}.$$
The mass contained in these particles cannot be greater than if every quantum state is filled by one fermion of mass $m$ and if this is to explain dark matter, then this mass must be $\sim M$. Thus
$$M &lt; m\left(\frac{4\pi r^3}{3}\right) \left(\frac{8\pi}{3}\right) \left(\frac{m\sqrt{2GM/r}}{h}\right)^3$$
and
$$mc^2 &gt; 8.9\left(\frac{r}{\rm 100 kpc}\right)^{-3/8} \left(\frac{M}{10^{12}M_\odot}\right)^{-1/8}\ {\rm eV}\ .$$
Thus there are not enough quantum states to accommodate a halo of fermions unless their rest mass energies exceed about 10 eV. For neutrinos, there are 3 flavours and anti particles, which reduces this number by $6^{1/4}$, but conversely it must be increased because particles in the halo cannot be uniformly distributed in velocity between 0 and $v$.
10 ev is about two orders of magnitude larger than the likely rest masses of the known neutrinos.
The idea that neutrinos from stars can make any contribution to dark matter halos is untenable. The vast majority of solar neutrinos have energies above 0.1 MeV, and so for an assumed neutrino rest mass energy of $\sim 0.1$ eV, they have Lorentz factors that exceed $10^6$ - i.e. they travel exceedingly close to the speed of light and are not confined to galaxies. The neutrinos emitted during supernova explosions are even more energetic. Secondly, even if you invent a magical process that can produce neutrinos with kinetic energies less than 0.1 eV, you still have the Tremaine-Gunn limit to overcome and even if all the rest mass of all the stars in a galaxy was converted into neutrinos, that mass would still fall short by more than an order of magnitude of that required to explain dark matter in galaxies. You cannot imagine that the neutrinos would &quot;pile up&quot; over time because the vast majority of the mass that has ever been turned into stars is still in the form of stars today and the stars that have already lived and died are a tiny percentage of the required dark matter mass.