Immaterium

The Immaterium (more commonly known as the Warp) is an alternate dimension in the Warhammer 40,000 setting.

Interaction with the Physical World

The Warp is notable for being influenced by the minds of most sapient beings. This interaction ranges from subtle effects of conscious or subconscious thought inside the Warp to conscious manipulation of Warp energies as psionic powers. This interaction is also harmful, however, as powerful emotions from large groups of individuals can give rise to entities inside the warp, generally referred to as daemons.

The most common use of the Warp by the various races of the Milky Way galaxy is FTL travel. Warp travel is the basic form of FTL travel in the Warhammer universe, but it has numerous problems. The Warp is an unstable place and is filled with various hazards including warp storms and daemons. To navigate in the Warp, the Imperium created the Astronomican as a psionic beacon, which psychic navigators can see and use as a reference point. The Tau, lacking psychic abilities, are only capable of making short jumps throught the Warp, a few light-years at a time.

The C'tan and their Necron minions desire to permanently isolate the Warp from the physical world.