From Wikitroid, the Metroid wiki

The Light of Aether is a mysterious energy source used by the Luminoth to sustain their settlements in Agon, Torvus, and Sanctuary. After their war with the Ing, the Light of Aether was stolen from them and taken to the Sky Temple, to the Emperor. There it remained until Samus Aran took it back to the Great Temple, therefore crippling Dark Aether.When the Leviathan collided with Aether it split the Light of Aether to both the dark and light world.