What is the name of the dark dimension in Metroid Prime 2: Echoes?

The dark dimension in Metroid Prime 2: Echoes is called Dark Aether.

Dark Aether is a parallel version of the planet Aether, created after a Phazon meteor struck the world and split its planetary energy. The impact produced a dimensional counterpart with a hostile environment and a poisonous atmosphere. The native Ing emerged from this dark dimension and waged war against the Luminoth of Aether.

Samus must repeatedly move between Light Aether and Dark Aether because the two worlds share a broad layout but contain different enemies, hazards, and paths. In Dark Aether, she needs safe zones to avoid the atmosphere’s damage; later suit upgrades reduce or eliminate that danger.

Dark Aether is not Tallon IV, the planet featured in Metroid Prime, nor SR388, the Metroid species’ original homeworld. Phaaze is a separate Phazon-producing planet introduced in Metroid Prime 3: Corruption. The game’s Japanese title, Metroid Prime 2: Dark Echoes, reflects the central light-and-dark setting.