  We point out a novel role for the Standard Model neutrino in dark matter
phenomenology where the exchange of neutrinos generates a long-range potential
between dark matter particles. The resulting dark matter self interaction could
be sufficiently strong to impact small-scale structure formation, without the
need of any dark force carrier. This is a generic feature of theories where
dark matter couples to the visible sector through the neutrino portal. It is
highly testable with improved decay rate measurements at future $Z$, Higgs, and
$\tau$ factories, as well as precision cosmology.
