We discuss the viability of a light particle (∼30 eV neutrino) with
strong self-interactions as a dark matter candidate. The interaction prevents
the neutrinos from free-streaming during the radiation dominated regime so
galaxy sized density perturbations can survive. Smaller scale perturbations are
damped due to neutrino diffusion. We calculate the power spectrum in the
imperfect fluid approximation, and show that it is damped at the length scale
one would estimate due to neutrino diffusion. The strength of the
neutrino--neutrino coupling is only weakly constrained by observations, and
could be chosen by fitting the power spectrum to the observed amplitude of
matter density perturbations. The main shortcoming of our model is that
interacting neutrinos can not provide the dark matter in dwarf galaxies.Comment: 17 pages, Latex, accepted for publication in Phys. Rev.
