  In 15 years of data taking the Pierre Auger Observatory has observed no
events beyond $10^{11.3}$ GeV. This null result translates into an upper bound
on the flux of ultrahigh-energy cosmic rays implying $J (> 10^{11.3}$ GeV) $<
3.6 \times 10^{-5}$ km$^{-2}$ sr$^{-1}$ yr$^{-1}$, at the 90\%C.L. We interpret
this bound as a constraint on extreme-energy photons originating in the decay
super-heavy dark matter (SHDM) particles clustered in the Galactic halo. Armed
with this constraint we derive the strongest lower limit on the lifetime of
hadronically decaying SHDM particles with masses in the range, $10^{14} < M_X/
$ GeV $< 10^{15}$. We also explore the capability of future NASA's POEMMA
mission to search for SHDM signals.
