  We study the cosmological evolution and phenomenological properties of scalar
bosons in the keV to MeV range that have a tiny mixing with the Standard Model
Higgs boson. The mixing determines both the abundance of light scalars produced
via the freeze-in mechanism and their lifetime. Intriguingly, the parameters
required for such scalars to account for all of the dark matter in the present
Universe generically predict lifetimes comparable to the sensitivity of present
and future indirect detection experiments. In order to accurately determine the
relic abundance of light scalars, we calculate freeze-in yields including
effects from finite temperatures and quantum statistics and develop a new
approach for solving the Boltzmann equation for number-changing processes in
the dark sector. We find that light scalars can potentially explain the
anomalous x-ray emission at 3.5 keV, while evading constraints from structure
formation and predicting potentially observable self-interaction cross
sections.
