At the dawn of time, there was nothing. Then, from the void appeared the Great Goddess, incarnation of Wakfu, and the Great Dragon, incarnation of Stasis. Being literally alone in the universe, they befriended each other and started to dance, more and more passionately.
From their dance and their love, an egg was created (as is common with dragons) - but this egg was an entire universe: the Krosmoz, filled with galaxies and solar systems.
From this union, Wakfu, the energy of creation, and Stasis, the energy of destruction, became forever linked. At the edges of the egg-universe, Wakfu enters the planes (or dimensions) through Incarnam, where souls are incarnated, while Stasis enters through Externam, the realm of the dead.
As the diagram opposite shows, other planes exist within the egg. The outer layer contains Ingloriom, the plane that was chosen by the gods as their residence, and the Shustuft Crust, home of demons. Separating the two is the Astral Plane, and within it the Aether Plane. Inside, the Material Plane is found, where the known world is found. It contains innumerable solar systems and worlds, one of them being the World of Twelve, where the stories of Dofus and Wakfu take place.
"Universe" and "universe"
Some sources, such as Dofus Artbook Session 2 and Dofus Mag 1, distinguish between "Universe" and "universe" (with a little "u," as they like to specify). Following the interpretation above, the Universe may correspond to Ingloriom, and the universe to the Shustuft Crust, but the Material plane introduces confusion between the two interpretations.
Dofus Mag 1 explains that the two universes were once separate, until Mongrelamus inadvertently ripped the veil between the two. Dofus Artbook Session 2 goes into further detail and indicates that he was the one who planted the sign that read "There's nothing here" in burning letters, which was later found by Osamodas. Regardless, the two universes became linked, allowing demons and gods to cross from one to the other, until they made a pact to forbid crossing from one universe to the other. That pact is most commonly referred to as what prevents demons from invading the World of Twelve.
The magazine defines the Universe as worlds of light, love, glory and beauty. The universe, on the other hand, contains worlds of darkness, where dwell demons, damned souls and ghosts. It also mentions that the World of Twelve was once part of the Universe, but doesn't elaborate on when and why that situation changed, or where the World of Twelve now stands. Perhaps it has transitioned from Ingloriom to the Material plane, but it may be that the universes and the planes are simply two incompatible interpretations of the Krosmoz.