A Parallel World in the Dark

Understand

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.