The Materium, also called realspace, is the four-dimensional region of space-time where corporeal beings live; it is the physical plane of existence defined by the four basic forms of physical energy and the basic elements of matter, as opposed to the Immaterium, more often called the "Warp" or "Warpspace," the hyperdimensional, non-Euclidean, psychically-reactive region of space-time that underlies the physical universe. The Warp is both a mirror of realspace while at the same time permeating all of it as if it were an invisible, undetectable mist.
The Materium abides by all the laws of physics and the natural world that we know today. The Warp, on the other hand, is another dimension where daemons and other Warp entities like Enslavers and Psychneuein exist, created and maintained by the collective psychic unconsciousness of the sentient beings of the material universe, particularly Mankind, since humanity is the galaxy's most populous and dominant intelligent species in the late 41st Millennium. These Warp entities lust to cross over into the Materium to create havoc and consume the souls of mortals.
Travel through the Materium is relatively slow and the greatest achievable speed is that of the speed of light in a vacuum (lightspeed). Therefore, many intelligent, starfaring species choose to travel through the Warp using Warp-Drives or similar technologies like the Eldar Webway. Because of the non-Euclidean nature of Warpspace, it is possible for a starship to travel dozens or even hundreds of light years across realspace in a short period of relative time while moving only a fraction of that distance in the Warp.
Certain regions within the Milky Way Galaxy exist where the Warp and the Materium essentially bleed into one another due to an enormous trauma to the underlying boundaries of space-time, creating the permanent Warp Storms known as Warp rifts. The most infamous of these rifts are the Eye of Terror in the Segmentum Obscurus and the Maelstrom in the Ultima Segmentum in the galactic east.
Since the birth of the Great Rift in 999.M41, the veil between the Materium and the Warp has grown progressively more strained, allowing Warp entities to make their way into realspace with much greater ease. In some places with heavy influence from the inhabitants of the Warp, such as the Scourge Stars controlled by the servants of the Plague God Nurgle, the environments of the Materium and Immaterium have begun to merge, much as they long have within Warp rifts like the Eye of Terror and the Maelstrom. It is likely that unless the waxing power of Chaos can be contained, that the outbreaks of Warp activity into realspace will only become worse, ultimately threatening the very nature of reality itself.
Sources
- Codex Adeptus Astartes - Space Marines (8th Edition), pp. 16-17
- Codex: Chaos Daemons (6th Edition), pp. 6-17
- Codex: Chaos Daemons (4th Edition), pp. 4-13
- White Dwarf 139 (UK), pg. 139
- Warhammer Community - New Warhammer 40,000: The Great Rift