  The diffuse gamma ray emission from astrophysical backgrounds in our Galaxy
and the signal due to the annihilation or decay of Dark Matter (DM) in the
Galactic Halo are expected to have a substantially different morphology and
spectral signatures. In order to exploit this feature we perform a full sky and
spectral binned likelihood fit of both components, using data collected during
the first 21 months of operation of the Fermi-LAT observatory. Preliminary
constraints are presented on the DM annihilation cross section and decaying
rate for various masses and annihilation/decay modes.
