  We analyze the indirect astrophysical signatures of secluded models of WIMP
dark matter, characterized by a weak-scale rate for annihilation into light
MeV-scale mediators which are metastable to decay into Standard Model states.
Such scenarios allow a significant enhancement of the annihilation cross
section in the galactic halo relative to its value at freeze-out, particularly
when the mediator is light enough for this process to proceed through radiative
capture to a metastable `WIMP-onium' bound state. For MeV-scale vector
mediators charged under a hidden U(1)' gauge group, the enhanced annihilation
rate leads predominantly to a sizable excess positron flux, even in the absence
of astrophysical boost factors.
