  We consider the possibility that the majority of dark matter in our Universe
consists of black holes of primordial origin. We determine the conditions under
which such black holes may have originated from a single-field model of
inflation characterized by a quartic polynomial potential. We also explore the
effect of higher-dimensional operators. The large power spectrum of curvature
perturbations that is needed for a large black hole abundance sources sizable
second order tensor perturbations. The resulting stochastic background of
primordial gravitational waves could be detected by the future space-based
observatories LISA and DECIGO or --as long as we give up on the dark matter
connection--by the ground-based Advanced LIGO-Virgo detector network.
