  We review the annihilation of dark matter into neutrinos over a range of dark
matter masses from MeV$/c^2$ to ZeV$/c^2$. Thermally-produced models of dark
matter are expected to self-annihilate to standard model products. As no such
signal has yet been detected, we turn to neutrino detectors to constrain the
``most invisible channel.'' We review the experimental techniques that are used
to detect neutrinos, and revisit the expected contributions to the neutrino
flux at current and upcoming neutrino experiments. We place updated constraints
on the dark matter self-annhilation cross section to neutrinos $\langle \sigma
v \rangle$ using the most recently available data, and forecast the sensitivity
of upcoming experiments such as Hyper-Kamiokande, DUNE, and IceCube Gen-2.
Where possible, limits and projections are scaled to a single set of dark
matter halo parameters for consistent comparison. We consider Galactic and
extragalactic signals of $s$, $p$, and $d$-wave annihilation processes directly
into neutrino pairs, yielding constraints that range from $\langle \sigma v
\rangle \sim 2.5\times10^{-26}~{\rm cm}^3 {\rm s}^{-1}$ at 30 MeV$/c^2$ to
$10^{-17}~{\rm cm}^3{\rm s}^{-1}$ at 10$^{11}$ GeV$/c^2$. Experiments that
report directional and energy information of their events provide much stronger
constraints, outlining the importance of making such data public.
