Converting non-relativistic dark matter to radiation
  Dark matter in the cosmological concordance model is parameterised by a
single number, describing the covariantly conserved energy density of a
non-relativistic fluid. Here we test this assumption in a model-independent and
conservative way by considering the possibility that, at any point during the
cosmological evolution, dark matter may be converted into a non-interacting
form of radiation. This scenario encompasses, but is more general than, the
cases where dark matter decays or annihilates into these states. We show that
observations of the cosmic microwave background allow to strongly constrain
this scenario for any conversion time after big bang nucleosynthesis. We
discuss in detail, both from a Bayesian and frequentist point of view, in which
sense adding large-scale structure observations may even provide a certain
preference for a conversion of dark matter to radiation at late times. Finally
we apply our general results to a specific particle physics realisation of such
a scenario, featuring late kinetic decoupling and Sommerfeld-enhanced dark
matter annihilation. We identify a small part of parameter space that both
mitigates the tension between cosmic microwave and large-scale structure data
and allows for velocity-dependent dark matter self-interactions strong enough
to address the small-scale problems of structure formation.
