  The nature of dark matter remains one of the biggest open questions in
physics. Intriguingly, it has been suggested that dark matter may be explained
by another recently observed phenomenon: the detection of gravitational waves
by LIGO. LIGO's detection of gravitational waves from merging stellar mass
black holes renewed attention toward the possibility that dark matter consists
solely of black holes created in the very early universe and that these
primordial black holes are what LIGO is presently observing. Subsequent work on
this topic has ruled out the possibility that dark matter could consist solely
of black holes similar to those that LIGO has detected with masses above 10
solar masses. However, LIGO's connection to dark matter remains an open
question and in this work we consider a distribution of primordial black holes
that accounts for all of the dark matter, is consistent with LIGO's
observations arising from primordial black hole binaries, and resolves tension
in previous surveys of microlensing events in the Milky Way halo. The
primordial black hole mass distribution that we consider offers an important
prediction--LIGO may detect black holes smaller than have ever been observed
with ~1% of the black holes it detects having a mass less than the mass of our
Sun. Approximately one year of operating advanced LIGO at design sensitivity
should be adequate to begin to see a hint of a primordial black hole mass
distribution. Detecting primordial black hole binaries below a solar mass will
be readily distinguishable from other known compact binary systems, thereby
providing an unambiguous observational window for advanced LIGO to pin down the
nature of dark matter.
