Immaterium

The Immaterium is an alternate dimension, also 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 is far from being safe place to travel through. Great currents and storms can blow starships into uncharted areas of the galaxy and make travel through certain regions impossible due to warp storms as they could last centuries. Ships could find themselves becalmed in the immaterium, a terrible fate for it's passengers as they become playthings for the dark creatures that inhabit that diabolical realm. Sometimes a ship will emerge from warp space centuries after they left yet only have lived a day onboard, many more just vanish without trace. 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 but Psykers risk danger when travelling the Warp. As the Warp gives Psykers their powers by way of channeling its energy, this 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.