Molecular Clouds as the Origin of the Fermi Gamma-Ray GeV-Excess
  The so-called "GeV-excess" of the diffuse Galactic gamma-ray emission is
studied with a spectral template fit based on energy spectra. The spectral
templates can be obtained in a data-driven way from the gamma-ray data, which
avoids the use of emissivity models to subtract the standardbackground
processes from the data. Instead, one can determine these backgrounds
simultaneously with any "signals" in any sky direction, including the Galactic
disk and the Galactic center. Using the spectral template fit two hypothesis of
the "GeV-excess" were tested: the dark matter (DM) hypothesis assuming the
excess is caused by DM annihilation and the molecular cloud (MC) hypothesis
assuming the "GeV-excess" is related to a depletion of gamma-rays below 2 GeV,
as is directly observed in the Central Molecular Zone (CMZ). Both hypotheses
provide acceptable fits, if one considers a limited field-of-view centered
within 20$^\circ$ around the Galactic center and applies cuts on the energy
range and/or excludes low latitudes, cuts typically applied by the proponents
of the DM hypothesis. However, if one considers the whole gamma-ray sky and
includes gamma-ray energies up to 100 GeV we find that the MC hypothesis is
preferred over the DM hypothesis for several reasons: i) The MC hypothesis
provides significantly better fits; ii) The morphology of the "GeV-excess"
follows the morphology of the CO-maps, a tracer of MCs, i.e. there exists a
strong "GeV-excess" in the Galactic disk also at large longitudes; iii) The
massive CMZ with a rectangular field-of-view of $l \times b = 3.5^{\circ}
\times 0.5^{\circ}$ shows the maximum of the energy flux per log bin in the
diffuse gamma-ray spectrum at 2 GeV, i.e. the "GeV-excess", already in the raw
data without any analysis. The rectangular profile contradicts the spherical
morphology expected for DM annihilation.
