The Immaterium (also referred to as the Empyrean, the Aether, the Sea of Souls, the Realm of Chaos, Warpspace or most commonly, the Warp) is an alternate dimension of purely psychic energy that echoes and underlies the familiar four dimensions of the material universe. It is the source of all psychic powers and known instances of so-called "sorcery" or "magic" as well as the home dimension of the Chaos Gods and their myriad daemonic servants. In fact, the terms "Chaos" and "the Warp" are often used interchangeably by those aware of their existence within the Imperium of Man. Superficially, the Immaterium is Mankind's solution to the problem of faster-than-light travel. This function as an FTL medium for interstellar travel is achieved because the Immaterium is an extradimensional domain of pure psychic energy, with voidcraft navigating between its currents, as in an ocean.
The psychic energy that makes up the Immaterium is believed to be the direct result of the existence of sentience in the universe, in particular the intelligent species of the Milky Way Galaxy. Considered to be a dark reflection of the material universe, the Warp is an ocean of chaotic psychic energy, raw emotion given physical form. Stirred by strong emotion and action, the Immaterium is the true Realm of Chaos, home to the Dark Gods who comprise the Ruinous Powers and their legions of daemonic followers. The Immaterium is also rumoured by many cultures, human and xenos alike, to be the final resting place of the spirits of the dead, and therefore can be considered the "Underworld" of the Warhammer 40,000 universe.