  An interaction between dark matter and dark energy is usually introduced by a
phenomenological modification of the matter conservation equations, while the
Einstein equations are left unchanged. Starting from some general and
fundamental considerations, in this work it is shown that a coupling in the
dark sector is likely to introduce new terms also in the gravitational
dynamics. Specifically in the cosmological background equations a bulk
dissipative pressure, characterizing viscous effects and able to suppress
structure formation at small scales, should appear from the dark coupling. At
the level of the perturbations the analysis presented in this work reveals
instead the difficulties in properly defining the dark sector interaction from
a phenomenological perspective.
