Implications of the observation of dark matter self-interactions for
  singlet scalar dark matter
  Evidence for dark matter self-interactions has recently been reported based
on the observation of a spatial offset between the dark matter halo and the
stars in a galaxy in the cluster Abell 3827. Interpreting the offset as due to
dark matter self-interactions leads to a cross section measurement of
sigma_DM/m ~ (1-1.5) cm^2/g, where m is the mass of the dark matter particle.
We use this observation to constrain singlet scalar dark matter coupled to the
Standard Model and to two-Higgs-doublet models. We show that the most natural
scenario in this class of models is very light dark matter, below about 0.1
GeV, whose relic abundance is set by freeze-in, i.e., by slow production of
dark matter in the early universe via extremely tiny interactions with the
Higgs boson, never reaching thermal equilibrium. We also show that the dark
matter abundance can be established through the usual thermal freeze-out
mechanism in the singlet scalar extension of the Yukawa-aligned
two-Higgs-doublet model, but that it requires rather severe fine tuning of the
singlet scalar mass.
