Nebula

Nebula is a Halo 2 Anniversary multiplayer map intended for use as a Forge canvas. It is one of three skybox-only maps created for Forging; the playable area consists solely of an expanse of open space, surrounded by an asteroid belt. The map's skybox is identical to Halo 4's Impact Forge map.

Trivia
Along with sharing the skybox with Impact, the descriptions of the two maps may also imply they are the same location.