Our understanding of the -ray sky has improved dramatically in the past decade, however, the unresolved -ray background (UGRB) still has a potential wealth of information about the faintest -ray sources pervading the Universe. Statistical cross-correlations with tracers of cosmic structure can indirectly identify the populations that most characterize the -ray background. In this study, we analyze the angular correlation between the -ray background and the matter distribution in the Universe as traced by gravitational lensing, leveraging more than a decade of observations from the Fermi-Large Area Telescope (LAT) and 3 years of data from the Dark Energy Survey (DES). We detect a correlation at signal-to-noise ratio of 8.9. Most of the statistical significance comes from large scales, demonstrating, for the first time, that a substantial portion of the UGRB aligns with the mass clustering of the Universe as traced by weak lensing. Blazars provide a plausible explanation for this signal, especially if those contributing to the correlation reside in halos of large mass ( 1014 M ) and account for approximately 3040% of the UGRB above 10 GeV. Additionally, we observe a preference for a curved -ray energy spectrum, with a log-parabolic shape being favored over a power-law. We also discuss the possibility of modifications to the blazar model and the inclusion of additional -ray sources, such as star-forming galaxies, misalinged active galactic nuclei, or particle dark matter.
