One possibility that is being ignored is that high energy gamma rays could be residue from an alien communication system.
As technology advances, we have moved to using higher and higher frequency radiation to carry our communications.
Higher frequency systems can carry denser payloads of information, which is why fiberoptic systems, which use laser light, can carry far more information than any radio system.
Similarly, blu-ray discs are displacing older CD-Roms. Blu-ray discs use blue light, which is higher frequency than the red light used by CD-Roms. The higher frequency allows larger amounts of information to be packed onto the blu-ray discs.
Researchers are already looking to use ultra-violet for high density media. X-rays will be the next step after ultra-violet.
Advancing this trend to its logical conclusion, an advanced interstellar alien communication system could conceivably use hard gamma rays, generated and manipulated using technology we have yet to discover, to pack enormous payloads of information into their communications stream.
That could be the source of the anomalous hard gamma rays detected by observatories.