Credit: NASA/DOE/Fermi LAT Collaboration
Gamma-ray observations are not sensitive enough to clearly see the shape of the Moon's disk or any surface features... Fermi's Large Area Telescope (LAT) detects a prominent glow centred on the Moon's position in the sky.
Cosmic rays are mostly protons accelerated by some of the most energetic phenomena in the universe, like the blast waves of exploding stars and jets produced when matter falls into black holes.
Because the particles are electrically charged, they're strongly affected by magnetic fields, which the Moon lacks. As a result, even low-energy cosmic rays can reach the surface, turning the Moon into a handy space-based particle detector. When cosmic rays strike, they interact with the powdery surface of the Moon, called the regolith, to produce gamma-ray emission. The Moon absorbs most of these gamma rays, but some of them escape.
Seen at these energies, the Moon would never go through its monthly cycle of phases and would always look full.
Source: NASA/Goddard Space Flight Center. "Moon glows brighter than sun in images from NASA's Fermi." ScienceDaily. ScienceDaily, 15 August 2019. <www.sciencedaily.com/releases/2019/08/190815120656.htm>.