  We recently showed that postulated ultracompact minihalos with a steep
density profile do not form in realistic simulations with enhanced initial
perturbations. In this paper we assume that a small fraction of the dark matter
consists of primordial black holes (PBHs) and simulate the formation of
structures around them. We find that in this scenario halos with steep density
profiles do form, consistent with theoretical predictions. If the rest of the
dark matter consists of weakly interacting massive particles (WIMPs), we also
show that WIMPs in the dense innermost part of these halos would annihilate and
produce a detectable gamma-ray signal. The non-detection of this signal implies
that PBHs make up at most one billionth of the dark matter, provided that their
mass is greater than one millionth of the mass of the Sun. Similarly, a
detection of PBHs would imply that the remaining dark matter could not be
WIMPs.
