Gap Between Dimensions

The gap between dimensions is a location mentioned by Ganondorf in, after the defeat of his phantom. He is shown opening a portal to said location and casting the phantom inside.

Theories
As the precise nature of this dimension is unknown, there are multiple theories as to what it could be.
 * A Void: The simplest idea is that the gap between dimensions is nothing more than an empty void, likely outside of the Zelda universe altogether, or at least beyond the part in which the games occur. Some variations of this theory include the idea that this void separates Hyrule and Termina, which are widely believed to be separate universes.
 * The Dark World: In, the Dark World is frequently visited, and seems to be parallel in many ways to Hyrule. However, like the gap between dimensions, much about this dimension's nature is left unexplained. Though filled with monsters and, on occassion, people, none of these inhabitants seem to be natural denizens of the Dark World (indeed, the children of the Village of the Blue Maiden who are banished there return to the Light World after the defeat of Shadow Link, to name one example). Other characters, such as the Knights of Hyrule and potentially Link himself, were shown to be sent to the Dark World via the Trident, similarly to how Phantom Ganon was banished to the Gap following his defeat.
 * The Twilight Realm: Some believe that the gap between dimensions refers to the Twilight Realm. However, the Mirror of Twilight is stated to be the only link between the Light World and Twilight Realm. It is worth noting, however, that Zant, who was borrowing Ganondorf's power, seemed to be able to move freely between the two worlds, which could indicate that Triforce wielders (or at least the wielder of the Triforce of Power) can move and open portals freely between the worlds, thus allowing Ganondorf to banish his phantom there.

Trivia

 * In the German version of the game, Ganon does not make reference to a "gap between dimensions." Instead, he purports to banish his phantom to Hades, the mythical Greek underworld.