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 black hole at the center of the Andromeda Galaxy. Rojan claimed this bomb would force the black hole to expand, destroying the entire Andromeda Galaxy (including the Kelvan homeworld), as well as the M32 and M110 galaxies. However, the device Rojan launched was not an actual bomb, but a messenger probe containing a peace treaty between the Federation and Kelvan Empire. It is thus unknown whether an actual galaxy bomb exists. (TOS - Strange New Worlds 9 short story: "Gone Native")