Recent x-ray observations have discovered that groups and clusters of galaxies contain many more baryons in intergalactic gas than in stars. If high-energy cosmic rays are present there with an average intensity comparable to that observed in our Galaxy, they could produce the extragalactic diffuse gamma radiation.

• Received 26 January 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.3052