  Galaxy clusters are dominated by dark matter, and may have a larger
proportion of surviving substructure than, e.g, field galaxies. Due to the
presence of galaxy clusters in relative proximity and their high dark matter
content, they are promising targets for the indirect detection of dark matter
via Gamma-rays. Indeed, dedicated studies of sets of up to 100 clusters have
been made previously, so far with no clear indication of a dark matter signal.
Here we report on Gamma-ray observations of some 26,000 galaxy clusters based
on Pass-7 Fermi Large Area Telescope (LAT) data, with clusters selected from
the Tully 2MASS Groups catalog. None of these clusters is significantly
detected in Gamma-rays, and we present Gamma-ray flux upper limits between 20
GeV and 500 GeV.
  We estimate the dark matter content of each of the clusters in these
catalogs, and constrain the dark matter annihilation cross section, by
analyzing Fermi-LAT data from the directions of the clusters. We set some of
the tightest cluster-based constraints to date on the annihilation of dark
matter particles with masses between 20 GeV and 500 GeV for annihilation to a
gamma-ray line. Our cluster based constraints are not yet as strong as bounds
placed using the Galactic Center, although an uncertainty still exists
regarding the "boost factor" from cluster substructure, where we have chosen a
rather conservative value. Our analysis, given this choice of possible boost,
is not yet sensitive enough to fully rule out typical realistic DM candidates,
especially if the gamma-ray line is not a dominant annihilation mode.
