  Primordial black holes (PBHs) are a viable candidate for dark matter if the
PBH masses are in the currently unconstrained "sublunar" mass range. We revisit
the possibility that PBHs were produced by nucleation of false vacuum bubbles
during inflation. We show that this scenario can produce a population of PBHs
that simultaneously accounts for all dark matter, explains the candidate event
in Subaru Hyper Suprime-Cam (HSC) data, and contains both heavy black holes as
observed by LIGO and very heavy seeds of supermassive black holes. We
demonstrate with numerical studies that future observations of HSC, as well as
other optical surveys, such as LSST, will be able to provide a definitive test
for this generic PBH formation mechanism if it is the dominant source of dark
matter.
