  This is one chapter of the collection of problems in cosmology, in which we
assemble the problems that concern one of the most distinctive features of
modern cosmology---the interaction in the Dark Sector. The evolution of any
broadly applied model is accompanied by multiple generalizations that aim to
resolve conceptual difficulties and to explain the ever-growing pool of
observational data. In the case of Standard Cosmological Model one of the most
promising directions of generalization is replacement of the cosmological
constant with a more complicated, dynamic, form of dark energy and
incorporation of interaction between the dark components---dark energy (DE) and
dark matter (DM). Typically, DE models are based on scalar fields minimally
coupled to gravity, and do not implement explicit coupling of the field to the
background DM. However, there is no fundamental reason for this assumption in
the absence of an underlying symmetry which would suppress the coupling. Given
that we do not know the true nature of either DE or DM, we cannot exclude the
possibility that there is some kind of coupling between them. Whereas
interactions between DE and normal matter particles are heavily constrained by
observations (e.g. in the solar system and gravitational experiments on Earth),
this is not the case for DM particles. In other words, it is possible for the
dark components to interact with each other while not being coupled to standard
model particles. Therefore, the possibility of DE-DM interaction should be
investigated with utmost gravity.
  This version contains only formulations of 117 problems. The full collection,
with solutions included, is available in the form of a wiki-based resource at
universeinproblems.com. The cosmological community is welcome to contribute to
its development.
