  If even a relatively small number of black holes were created in the early
universe, they will constitute an increasingly large fraction of the total
energy density as space expands. It is thus well-motivated to consider
scenarios in which the early universe included an era in which primordial black
holes dominated the total energy density. Within this context, we consider
Hawking radiation as a mechanism to produce both dark radiation and dark
matter. If the early universe included a black hole dominated era, we find that
Hawking radiation will produce dark radiation at a level $\Delta N_{\rm eff}
\sim 0.03-0.2$ for each light and decoupled species of spin 0, 1/2, or 1. This
range is well suited to relax the tension between late and early-time Hubble
determinations, and is within the reach of upcoming CMB experiments. The dark
matter could also originate as Hawking radiation in a black hole dominated
early universe, although such dark matter candidates must be very heavy
($m_{\rm DM} >10^{11}$ GeV) if they are to avoid exceeding the measured
abundance.
