  Gravitinos are very promising candidates for the cold dark matter of the
Universe. Interestingly, to achieve a sufficiently long gravitino lifetime,
R-parity conservation is not required, thus preventing any dangerous
cosmological influence of the next-to-lightest supersymmetric particle. When
R-parity is violated, gravitinos decay into photons and other particles with a
lifetime much longer than the age of the Universe, producing a diffuse gamma
ray flux with a characteristic spectrum that could be measured in future
experiments, like GLAST or AMS-02. In this letter we compute the energy
spectrum of photons from gravitino decay and discuss its main qualitative
features.
