  We report the detection of a cross-correlation signal between {\it Fermi}
Large Area Telescope diffuse gamma-ray maps and catalogs of clusters. In our
analysis, we considered three different catalogs: WHL12, redMaPPer and
PlanckSZ. They all show a positive correlation with different amplitudes,
related to the average mass of the objects in each catalog, which also sets the
catalog bias. The signal detection is confirmed by the results of a stacking
analysis. The cross-correlation signal extends to rather large angular scales,
around 1 degree, that correspond, at the typical redshift of the clusters in
these catalogs, to a few to tens of Mpc, i.e. the typical scale-length of the
large scale structures in the Universe. Most likely this signal is contributed
by the cumulative emission from AGNs associated to the filamentary structures
that converge toward the high peaks of the matter density field in which galaxy
clusters reside. In addition, our analysis reveals the presence of a second
component, more compact in size and compatible with a point-like emission from
within individual clusters. At present, we cannot distinguish between the two
most likely interpretations for such a signal, i.e. whether it is produced by
AGNs inside clusters or if it is a diffuse gamma-ray emission from the
intra-cluster medium. We argue that this latter, intriguing, hypothesis might
be tested by applying this technique to a low redshift large mass cluster
sample.
