  Primordial black holes (PBHs) in the mass range $(30$--$100)~M_{\odot}$ are
interesting candidates for dark matter, as they sit in a narrow window between
microlensing and cosmic microwave background constraints. There are however
tight constraints from the binary merger rate observed by the LIGO and Virgo
experiments. In deriving these constraints, PBHs were treated as point
Schwarzschild masses, while the more careful analysis in an expanding universe
we present here, leads to a time-dependent mass. This implies a stricter set of
conditions for a black hole binary to form and means that black holes coalesce
much more quickly than was previously calculated, namely well before the
LIGO/Virgo's observed mergers. The observed binaries are those coalescing
within galactic halos, with a merger rate consistent with data. This reopens
the possibility for dark matter in the form of LIGO-mass PBHs.
