  The baryon-dark matter coincidence is a long-standing issue. Interestingly,
the recent observations suggest the presence of dark radiation, which, if
confirmed, would pose another coincidence problem of why the density of dark
radiation is comparable to that of photons. These striking coincidences may be
traced back to the dark sector with particle contents and interactions that are
quite similar, if not identical, to the standard model: a dark parallel world.
It naturally solves the coincidence problems of dark matter and dark radiation,
and predicts a sterile neutrino(s) with mass of ${\cal O}(0.1 - 1)$\,eV, as
well as self-interacting dark matter made of the counterpart of ordinary
baryons. We find a robust prediction for the relation between the abundance of
dark radiation and the sterile neutrino, which can serve as the smoking-gun
evidence of the dark parallel world.
