The Tibet ASγ experiment, a China-Japan joint research project on cosmic-ray observation, has discovered ultra-high-energy diffuse gamma rays from the Milky Way galaxy. The highest energy detected is estimated to be unprecedentedly high, nearly 1 Peta electronvolts. These gamma rays are spread out across the Milky Way. Scientists believe they are produced by the nuclear interaction between cosmic rays escaping from the most powerful galactic sources (''PeVatrons'') and interstellar gas in the Milky Way galaxy.