  Present and upcoming neutrino experiments can have considerable sensitivity
to dark sectors that interact feebly with the Standard Model. We consider dark
sectors interacting with the SM through irrelevant portals that are motivated
on general principles. We derive bounds on such scenarios by considering decays
of dark sector excitations inside the neutrino detector, placed downstream from
the target. Our approach is model agnostic and applies to a wide range of dark
sector models, both strongly and weakly coupled. In this approach, the dark
sector is characterized by two scales: $\Lambda_\text{UV}$ (mass of mediators
generating the portals) and $\Lambda_\text{IR}$ (mass gap of the dark sector).
At intermediate energies, far away from these scales, the theory is
approximately scale-invariant. This allows the calculation of production rates
independent of the threshold corrections, although some mild model-dependent
assumptions are needed. We look at various dark sector production processes
relevant at neutrino experiments such as meson decays, direct partonic
production, and dark bremsstrahlung. We consider representative experiments
from past (CHARM), present (ICARUS, NOvA, MicroBooNE), and upcoming future
(DUNE-MPD), and compare their reach to existing bounds from high energy
experiments (LHC and LEP) and dedicated future LLP experiments (SHiP). We find
that the upcoming DUNE-MPD can probe $\Lambda_\text{UV}$ in the TeV range, and
$\Lambda_\text{IR}$ in the 0.1-1 GeV range, covering parts of parameter space
currently inaccessible in high energy experiments and fixed-target/beam-dump
experiments, and is comparable to future LLP experiments. In general, future
neutrino experiments can be an efficient probe of dark sectors, providing
complementary as well as new reach in parameter space.
