  A simple and elegant mechanism is proposed to resolve the problem of having a
light scalar mediator for self-interacting dark matter and the resulting
disruption to the cosmic microwave background (CMB) at late times by the
former's enhanced Sommerfeld production and decay. The crucial idea is to have
Dirac neutrinos with the conservation of U(1) lepton number extended to the
dark sector. The simplest scenario consists of scalar or fermion dark matter
with unit lepton number accompanied by a light scalar dilepton mediator, which
decays to two neutrinos.
