The EM spectrum energetically ends with gamma rays, with wavelengths less than 10 picometers (10–12 meters): less than the diameter of an atom.
In deep space, gamma ray bursts occur regularly. The Sun powers low-energy gamma rays (up to 1010 eV). Midrange rays are energized by the shock waves of supernovae. The origin of high-energy rays, above 1015 eV, is a mystery.
Planet-side, radioactive atoms and nuclear blasts create gamma rays. Gamma rays readily kill living cells.