  The region around the Galactic center (GC) is now well established to be
brighter at energies of a few GeV than expected from conventional models of
diffuse gamma-ray emission and catalogs of known gamma-ray sources. We study
the GeV excess using 6.5 years of data from the Fermi Large Area Telescope. We
characterize the uncertainty of the GC excess spectrum and morphology due to
uncertainties in cosmic-ray source distributions and propagation, uncertainties
in the distribution of interstellar gas in the Milky Way, and uncertainties due
to a potential contribution from the Fermi bubbles. We also evaluate
uncertainties in the excess properties due to resolved point sources of gamma
rays. The Galactic center is of particular interest as it would be expected to
have the brightest signal from annihilation of weakly interacting massive dark
matter particles. However, control regions along the Galactic plane, where a
dark-matter signal is not expected, show excesses of similar amplitude relative
to the local background. Based on the magnitude of the systematic
uncertainties, we conservatively report upper limits for the annihilation cross
section as function of particle mass and annihilation channel.
