Despicable Dark Relics: generated by gravity with unconstrained masses
  We demonstrate the existence of a generic, efficient and purely gravitational
channel producing a significant abundance of dark relics during reheating after
the end of inflation. The mechanism is present for any inert scalar with the
non-minimal curvature coupling $\xi R\chi^2$ and the relic production is
efficient for natural values $\xi = {\cal O}(1)$. The observed dark matter
abundance can be reached for a broad range of relic masses extending from $m
\sim 1 {\rm k eV}$ to $m \sim 10^{8} {\rm GeV}$, depending on the scale of
inflation and the dark sector couplings. Frustratingly, such relics escape
direct, indirect and collider searches since no non-gravitational couplings to
visible matter are needed.
