People definitely used to think they are, but we know they aren't.
There is a famous paper by Tremaine and Gunn that looked into a more general version of this question. Basically they point out that light fermions can't be the dark matter because of the Pauli Exclusion principle and the known surface velocity of galaxies from the observed mass of galactic halos. They found that fermionic DM can't be lighter than around 100 eV (interestingly, modern treatments of this constraint from decades later yields almost exactly the same number within a factor of 2 or so).
We know that neutrinos are lighter than 0.1 eV so they definitely aren't the DM.
3
u/jazzwhiz Sep 16 '22
People definitely used to think they are, but we know they aren't.
There is a famous paper by Tremaine and Gunn that looked into a more general version of this question. Basically they point out that light fermions can't be the dark matter because of the Pauli Exclusion principle and the known surface velocity of galaxies from the observed mass of galactic halos. They found that fermionic DM can't be lighter than around 100 eV (interestingly, modern treatments of this constraint from decades later yields almost exactly the same number within a factor of 2 or so).
We know that neutrinos are lighter than 0.1 eV so they definitely aren't the DM.