  We consider the possibility that the black-hole (BH) binary detected by LIGO
may be a signature of dark matter. Interestingly enough, there remains a window
for masses $20\,M_\odot \lesssim M_{\rm bh} \lesssim 100\, M_\odot$ where
primordial black holes (PBHs) may constitute the dark matter. If two BHs in a
galactic halo pass sufficiently close, they radiate enough energy in
gravitational waves to become gravitationally bound. The bound BHs will rapidly
spiral inward due to emission of gravitational radiation and ultimately merge.
Uncertainties in the rate for such events arise from our imprecise knowledge of
the phase-space structure of galactic halos on the smallest scales. Still,
reasonable estimates span a range that overlaps the $2-53$ Gpc$^{-3}$ yr$^{-1}$
rate estimated from GW150914, thus raising the possibility that LIGO has
detected PBH dark matter. PBH mergers are likely to be distributed spatially
more like dark matter than luminous matter and have no optical nor neutrino
counterparts. They may be distinguished from mergers of BHs from more
traditional astrophysical sources through the observed mass spectrum, their
high ellipticities, or their stochastic gravitational wave background. Next
generation experiments will be invaluable in performing these tests.
