Dark Aether is a parallel dimension created on the planet Aether by a Leviathan impact. The dark dimension is the home of the Ing, a hostile species Samus Aran encounters in Metroid Prime 2: Echoes.
Details of Dark Aether's formation are not mentioned throughout Echoes. The Leviathan and the natural energy harvested by the Luminoth are somehow responsible for its creation. Landmarks on Aether and Dark Aether tend to mirror each other. A select few objects will actually move due to events in the other dimension.
Since Aether is split, its planetary energy is split as well. If one dimension gains control of all of the energy, the other perishes. A conflict over the energy provoked a lengthy war between the Luminoth and Ing, with the Luminoth on the losing side by the time Samus arrived. Energy is exchanged via the Energy Transfer Module, which binds to Samus after she defeats a Dark Alpha Splinter. Due to this bind and Samus's lack of a functional ship, she agrees to help the Luminoth leader, U-Mos, reclaim the lost energy in the different regions of Dark Aether. To access Dark Aether, Samus must use portals found scattered throughout Aether. These can either occur naturally or can be mechanically generated by Luminoth technology. The Ing can travel between Light and Dark Aether in a gaseous form, but must quickly possess a living creature to survive on Light Aether's surface.
Thanks to the efforts of Samus Aran, the Ing were defeated and the planetary energy they had harvested was transferred back to Light Aether. The removal of the energy from Dark Aether caused the bleak dimension to collapse into utter oblivion.