Across the Milky Way, charged particles race through space with energies beyond the reach of even the most powerful accelerators built on Earth. Known as cosmic rays, these particles consist mainly of protons, with a smaller number of electrons. “The highest energy of galactic cosmic rays can reach and exceed one quadrillion (1015) electron volts, or a peta electron volt (PeV). Gamma rays are the most energetic form of electromagnetic radiation and can be produced where cosmic rays are accelerated. Three clues identify proton accelerationThree pieces of evidence pointed to a proton PeVatron.