Other stuff. Many other dimensions exist. Gilgamesh obviously being the prime example of an outside coming in. However, the GI are from an entirely other dimension that they migrated from. The Cave of the GI is a boundary between the two worlds (@29:00). Gi stole the greatest materia of thousands of years ago, far predating Jenova's arrival. They did this because they could not join the afterlife. After they would die, they would just become wandering spirits that required true release. Having not been born of the Lifestream, they cannot return to it and thus suffer eternal penance. They took that materia and poured their collected sorrow, rage, and desires into it. This created the Black Materia. Which can truly give them the solace that they crave. It will destroy their spirits. However, it also has the possibility of destroying everything else (the world, souls, and the Lifestream) if used for that purpose. The moogles also live in (at least) little pocket...