There is overwhelming evidence that non-baryonic dark matter constitutes 23% of the energy density of the universe. Weakly Interacting Massive Particles are promising dark matter candidates that may produce monochromatic gamma rays via annihilation or decay. Such interactions would give a narrow spectral line in the Galactic diffuse gamma-ray energy spectrum. Our analysis searches for spectral lines from 5 GeV to 500 GeV using 4 years of Fermi LAT data.

For the detector response to a signal, we use a new two-dimensional probability distribution function that incorporates the quality of the gamma-ray energy measurement. In addition, given the many uncertainties associated with the galactic dark matter density distribution, we search in several different regions of interest optimized for various dark matter density profiles. Our results include 95% CL limits on the presence of gamma-ray lines as well as studies of systematic uncertainties and an evaluation of the robustness of the analysis method.