  In the post-LIGO era, there has been a lot of focus on primordial black holes
(PBHs) heavier than $\sim 10^{15}$g as potential dark matter (DM) candidates.
We point out that the branch of the PBH family that disappeared - PBHs lighter
than $\sim 10^9$g that ostensibly Hawking evaporated away in the early Universe
- also constitute an interesting frontier for DM physics. Hawking evaporation
itself serves as a portal through which such PBHs can illuminate new physics,
for example by emitting dark sector particles. Taking a simple DM scalar
singlet model as a template, we compute the abundance and mass of PBHs that
could have provided, by Hawking evaporation, the correct DM relic density. We
consider two classes of such PBHs: those originating from curvature
perturbations generated by inflation, and those originating from false vacuum
collapse during a first-order phase transition. For PBHs of both origins we
compute the gravitational wave (GW) signals emanating from their formation
stage: from second-order effects in the case of curvature perturbations, and
from sound waves in the case of phase transitions. The GW signals have peak
frequencies in the MHz-GHz range typical of such light PBHs. We compute the
strength of such GWs compatible with the observed DM relic density, and find
that the GW signal morphology can in principle allow one to distinguish between
the two PBH formation histories.
