  In the standard picture of stellar evolution, pair-instability -- the energy
loss in stellar cores due to electron-positron pair production -- is predicted
to prevent the collapse of massive stars into black holes with mass in the
range between approximately 50 and 130 solar masses -- a range known as the
"{\em black hole mass gap}". LIGO detection of black hole binary mergers
containing one or both black holes with masses in this {\em mass gap} thus
challenges the standard picture, possibly pointing to an unexpected merger
history, unanticipated or poorly understood astrophysical mechanisms, or new
physics. Here, we entertain the possibility that a "dark sector" exists,
consisting of dark electrons, dark protons, and electromagnetic-like
interactions, but no nuclear forces. Dark stars would inevitably form given
such dark sector constituents, possibly collapsing into black holes with masses
within the mass gap. We study in detail the cooling processes necessary for
successful stellar collapse in the dark sector and show that for suitable
choices of the particle masses, we indeed predict populating the mass gap with
dark sector black holes. In particular, we numerically find that the heavier of
the two dark sector massive particles cannot be lighter than, approximately,
the visible sector proton for the resulting dark sector black holes to have
masses within the mass gap. We discuss constraints on this scenario and how to
test it with future, larger black hole merger statistics.
