  We set conservative, robust constraints on the annihilation and decay of dark
matter into various Standard Model final states under various assumptions about
the distribution of the dark matter in the Milky Way halo. We use the inclusive
photon spectrum observed by the Fermi Gamma-ray Space Telescope through its
main instrument, the Large-Area Telescope (LAT). We use simulated data to first
find the "optimal" regions of interest in the gamma-ray sky, where the expected
dark matter signal is largest compared with the expected astrophysical
foregrounds. We then require the predicted dark matter signal to be less than
the observed photon counts in the a priori optimal regions. This yields a very
conservative constraint as we do not attempt to model or subtract astrophysical
foregrounds. The resulting limits are competitive with other existing limits,
and, for some final states with cuspy dark-matter distributions in the Galactic
Center region, disfavor the typical cross section required during freeze-out
for a weakly interacting massive particle (WIMP) to obtain the observed relic
abundance.
