Manifesting hidden dynamics of a sub-component dark matter
  We emphasize the distinctive cosmological dynamics in multi-component dark
matter scenarios and its impact in probing a sub-dominant component of dark
matter. We find that the thermal evolution of the sub-component dark matter is
significantly affected by the sizable self-scattering that is naturally
realized for sub-${\rm GeV}$ masses. The required annihilation cross section
for the sub-component sharply increases as we consider a smaller relative
abundance fraction among the dark-matter species. Therefore, contrary to a
naive expectation, it can be easier to detect the sub-component with smaller
abundance fractions in direct/indirect-detection experiments and cosmological
observations. Combining with the current results of accelerator-based
experiments, the abundance fractions smaller than $10\,\%$ are strongly
disfavored; we demonstrate this by taking a dark photon portal scenario as an
example. Nevertheless, for the abundance fraction larger than $10\,\%$, the
warm dark matter constraints on the sub-dominant component can be complementary
to the parameter space probed by accelerator-based experiments.
