The Great Dark Nebula is a region of the Universe containing many inhabited worlds.
The Galra Empire's campaign to conquer the nebula was underway by the mid-1980's. Many people were killed or enslaved. The conquest campaign was largely complete by 1999.
That same year, five earthlings were among the slaves in Galra, having been captured in the wake of World War III. They escaped to Planet Altea on the edge of the nebula, and with Princess Fala and Raible, brought back Golion. For about a year they fought against Galra until the empire fell.