  In the hierarchical paradigm of structure formation, galaxy clusters are the
largest objects ever to virialize. They are thought to grow by accreting mass
through large scale, strong virial shocks. Such a collisionless shock is
expected to accelerate relativistic electrons, thus generating a spectrally
flat leptonic virial ring. However attempts to detect virial rings have all
failed, leaving the shock paradigm unconfirmed. Here we identify a virial
$\gamma$-ray signal by stacking Fermi-LAT data for 112 clusters, enhancing the
ring sensitivity by rescaling clusters to their virial radii and utilizing the
anticipated spectrum. In addition to a central unresolved, hard signal
(detected at the nominal $5.8\sigma$ confidence level), probably dominated by
active galactic nuclei, we identify ($5.9\sigma$) a bright, spectrally flat
$\gamma$-ray ring at the expected shock position. It corresponds to $\sim
0.6\%$ (with an uncertainty factor $\sim2$) thermal energy deposition in
relativistic electrons over a Hubble time. This result validates the shock
paradigm, calibrates its parameters, and indicates that the cumulative emission
from such shocks significantly contributes to the diffuse extragalactic
$\gamma$-ray and radio backgrounds.
