The diffuse extragalactic gamma-ray background is the nearly isotropic high-energy glow that remains after subtracting resolved sources and Galactic foregrounds, and is thought to arise from the superposition of many unresolved populations such as blazars, radio galaxies, star-forming galaxies, clusters, and possibly more exotic processes (Fornasa & Sánchez-Conde 2015; Ackermann et al. 2015). ΛCDM-based source and structure-formation models can reproduce much of the measured spectrum but still face composition and anisotropy uncertainties, since explaining both the observed intensity and small-scale angular structure without overproducing any single population or invoking finely tuned dark-matter contributions remains challenging (Inoue 2014; Di Mauro & Donato 2015).