  Primordial black holes (PBHs) can constitute the predominant fraction of dark
matter (DM) if PBHs reside in the currently unconstrained "sublunar" mass
range. PBHs originating from scalar perturbations generated during inflation
can naturally appear with a broad spectrum in a class of models. The resulting
stochastic gravitational wave (GW) background generated from such PBH
production can account for the recently reported North American Nanohertz
Observatory for Gravitational Waves (NANOGrav) pulsar timing array data signal,
and will be testable in future GW observations by interferometer-type
experiments such as Laser Interferometer Space Antenna (LISA). We show that the
broad mass function of such PBH DM is already being probed by Subaru Hyper
Suprime-Cam (HSC) microlensing data and is consistent with a detected candidate
event. Upcoming observations of HSC will be able to provide an independent
definitive test of the stochastic GW signals originating from such PBH DM
production scenarios.
