The Immaterium is the Warhammer 40,000 universe's solution to interstaller travel. An alternate dimension called the Warp which is used to travel effectively faster than light. This is achieved by the Warp being a domain of pure energy, with spacecraft navigating between streams, as in an ocean. Time also proceeds a great deal slower in the Warp than in the Material universe, creating a form of stasis. While an effective solution to generations-long trips across the galaxy, it is a far from perfect solutions. Long-distance journeys take months or even years, and the timing itself is unpredictable due to the very nature of the Warp.

The Warp itself is home to a great many creatures, many of which are hostile to foreign objects from the Material universe. Ships travelling in the Warp are shielded against attacks from these creatures. It gives Psykers their powers by way of channeling its energy, which makes them vulnerable to attack from Warp entities and Daemons.

The Warp is also home to the four Chaos gods; Khorne, Slaanesh, Tzeentch, and Nurgle.

There are conflicting accounts as to whether it is Tzeentch or the Emperor of Mankind which prevents the warp spilling out and completely engulfing the material universe.