Darth Raizen
Genibus Nitito Canus
I have just made a discovery. In the English sub, the narrator said that a SSJ has destroyed the South Galaxy, but when I watch the movie more, King Kai said that he tore up most of the South Galaxy.
In the Japanese sub, at the start of the movie, King Kai said that a SSJ has attacked the South Galaxy, and then he said that it has just been destroyed.
So technically, he can destroy a galaxy, but I guess it depends on the level of destruction.
Also, I have looked further into this. Since it was stated that he can destroy the entire universe by Piccolo to Vegeta, he possibly can, but from what we know about DBZ, there are only 4 galaxies in their universe, North, South, East, and West, so technically, he can.
But I don't think he can destroy a universe such as ours, or others such as DC, as there are billions of galaxies in those universes, including ours.