  In this article we discuss a general effective-theory description of a
multi-component dark sector with an unspecified non-trivial symmetry and its
interactions with the Standard Model generated by the exchange of heavy
mediators. We then categorize the relevant effective operators given the
current experimental sensistivity where the underlying theory is weakly coupled
and renormalizable, with neutral mediators: either scalars or fermions. An
interesting scenario resulting from this categorization is the neutrino portal,
where only fermion mediators are present, and where the dark sector consists of
fermions and scalars such that the lightest dark particle is a fermion, this
scenario is characterized by having naturally suppressed couplings of the DM to
all SM particles except the neutrinos and has received little attention in the
literature. We find that there is a wide region in parameter space allowed by
the current experimental constraints (relic abundance, direct and indirect
detection limits); the cleanest signature of this paradigm is the presence of
monochromatic neutrino lines with energy equal to that of the DM mass, but
experimental sensitivity would have to be improved significantly before this
can be probed.
