A galaxy bomb was reportedly a massive destructive device that was capable of travelling galactic distances.
In the late 26th century, Kelvan Commander Rojan told his former superior, Prefect Tamar, that he had ordered a galaxy bomb to be launched from the Milky Way Galaxy to target the planet Kelva in the Andromeda Galaxy, after being branded a traitor by Tamar. However, the device was not actually a bomb; it was a formal peace treaty between Kelva and the Federation. It is thus unknown whether an actual galaxy bomb exists, as none have as yet appeared. (TOS - Strange New Worlds 9 short story: "Gone Native")