This article describes Subspace in general. For the actual stages, see Subspace (Part I) and Subspace (Part II).
The Midair Stadium being engulfed by Subspace.

Subspace (亜空間 Subspace) is a malevolent void-like dimension central to the plotline of Adventure Mode: The Subspace Emissary. The dimension's general appearance is a mottled and purplish-black sky with very little matching ground as lightning laces the sky.

The fighters in Subspace.

Subspace is enforced on the World of Trophies by Tabuu. Over the course of the Subspace Emissary, Subspace Bombs are used to remove portions of the world, drag them into Subspace and leave behind a bubble-like rift into the dimension in its place. While inactive fighters and plant life survive the detonation, it is likely fatal to any living creature, based on how nearby R.O.B. Sentries are torn apart. Once the initial Subspace Bomb detonation is complete, Subspace can be entered by simply walking in these "subspacial bubbles".

Once a piece of the world has been shunted into Subspace, it floats around as a bubble-like "colony". The colonies are then shut off from the rest of the world, Subspace and otherwise, unless a connection is made between the colony and either Subspace or another colony. Tabuu used the many pieces of the world he collected to create The Great Maze.

Kirby exploring Subspace.

Trivia

  • Despite sharing a name with the Subspace of Super Mario Bros. 2, this Subspace is unrelated. However, it is visually and thematically very similar to The Void of Super Paper Mario.
  • Subspace also shares, perhaps coincidentally, several similarities with the Void from Final Fantasy V: both are controlled by the main antagonist, in both cases consumed regions appear in the outside world like large spheres of darkness and the last area of both is a combination of several absorbed areas from earlier in the game and features many boss fights.