From Wikitroid, the Metroid wiki

Dark Aether was created when a Leviathan from Phaaze crashed onto Aether, splitting it into two dimensions. Dark Aether is called as such because it is a twisted "Dark" version of Aether and the "Dark Matter" in its atmosphere is damaging to "light" beings, such as humans, Luminoths, Space Pirates, and Splinter. Dark Aether is home to the Ing a race of shadow beings, their connection to Phazon is uncommon as they are seemingly unaffected by it's intense radiation. This is possibly because the Emperor Ing would have originally been a native creature that was corrupted to protect the Leviathan's core. However, it is just as possible that the Leviathan created the Dark Dimension and as a result, the Ing race which may be the "mirror" counterpart of the Luminoth that inhabited the planet. Dark Aether is only accessible from Aether by Dark Portals. Since the two planets share the same space, they vie for planetary energy. Aether and Dark Aether were at war for about 70 years until bounty hunter Samus Aran fought on the side of the Luminoth. Samus successfully removed the remaining planetary energy from Dark Aether, destroying the Dark Dimension in the process. The Dark Dimension collapsed on Dark Samus but she survived.

The creation of Dark Aether split Aether's planetary energy into two halves, leaving both planets weak and unstable.

It is interesting to note that Phazon is only found within Dark Aether. Luminoth Lore seems to suggest that after initial impact on Aether the Phazon Meteor opened a rift to the Dark Dimension, landing in Dark Aether. This is supported as the Luminoth Lore data says that after the Luminoth emerged from their shelters the Phazon Meteor or any substantial crater was no where to be seen. Planetary scans of Dark Aether reveal a large crater and deep cracks running through half of the Dark Planet.