Not quite black holes as dark matter
  Primordial black holes that survive until the present have been considered as
a dark matter candidate. In this paper we argue that primordial 2-2-hole
remnants provide a more promising and testable option. 2-2-holes arise in
quadratic gravity as a new family of classical solutions for ultracompact
matter distributions and they possess the black hole exterior without an event
horizon. They may serve as the endpoint of gravitational collapse, providing a
resolution for the information loss problem. Intriguing thermodynamic behavior
is found for these objects when sourced by a thermal gas. A large 2-2-hole
radiates with a Hawking-like temperature and exhibits an entropy-area law. At a
late stage, the evaporation slows down and essentially stops as the mass
asymptotically approaches a minimal value. This remnant mass is determined by a
fundamental scale in quadratic gravity. We study the cosmological and
astrophysical implications of having these remnants as dark matter and derive
the corresponding constraints. A distinctive phenomenon associated with remnant
mergers occurs, predicting fluxes of high-energy astrophysical particles due to
the spectacular evaporation of the merger product. Measurements of high-energy
photon and neutrino fluxes could possibly bound the remnant mass to be not far
above the Planck mass. Early-universe physics, on the other hand, requires that
2-2-holes quickly evolve into the remnant state after formation, putting an
upper bound on the formation mass.
