(Disclaimer: This bit of lore will contradict pre-established lore. Thoughts are still welcome.)

The Aetherium (also referred to as the Aether, Paradiso, Heaven, etc.) is a plane that exists above the material plane & separate from the Immaterium. It is a dimension ruled over by the one true God that is both connected & separate from the other planes. Unlike the Warp, which influences & is influenced by the psychic activity of mortals, the Aether is unchanging & unaffected by any other realms or powers, though it is stated that inhabitants of the Aether can interact & influence mortals much like how the warp does. It is noted, however, that influence & interaction of the Aether upon the material plane is too subtle for even those with immense spiritual awareness to fully detect. Because of this, most outside the Golden Kingdom simply attribute any Aether activity as originating from the Warp.