  The formation of stable or meta-stable bound states can dramatically affect
the phenomenology of dark matter (DM). Although the capture into bound states
via emission of a vector is known to be significant, the capture via scalar
emission suffers from cancellations that render it important only within narrow
parameter space. While this is true for neutral scalar mediators, here we show
that bound-state formation via emission of a charged scalar can be extremely
significant. To this end, we consider DM charged under a dark $U(1)$ force and
coupled also to a light complex scalar that is charged under the same gauge
symmetry. We compute the cross-sections for bound-state formation via emission
of the charged scalar, and show that they can exceed those for capture via
vector emission, as well as annihilation, by orders of magnitude. This holds
even for very small values of the DM coupling to the charged scalar, and
remains true in the limit of global symmetry. We then compute the DM thermal
freeze-out, and find that the capture into meta-stable bound states via
emission of a charged scalar can cause a late period of significant DM
depletion. Our results include analytical expressions in the Coulomb limit, and
are readily generalisable to non-Abelian interactions. We expect them to have
implications for Higgs-portal scenarios of multi-TeV WIMP DM, as well as
scenarios that feature dark Higgses or (darkly-)charged inert scalars,
including models of self-interacting DM.
