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 WH40K 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. To protect the crews of spacecraft from the threats of warp travel, Imperial starships employ a device known as a Gellar Field.

The Tau, lacking psychic abilities, are only capable of making short jumps through the Warp, a few light-years at a time.

There are areas in which the physical world and the Warp intersect; the most notable of them is known as the Eye of Terror, which is currently a stronghold for the myriad forces of Chaos.

The C'tan and their Necron minions have no need to make use of the warp, having FTL technology that doesn't rely upon it. They desire to permanently isolate the Warp from the physical world.