A fractal core is a convergence of extreme gravitational forces, formed by significant concentrations of mass such as stars, singularities, or simply any large enough quantities of converging matter. In appearance a fractal core is a swirling vortex that most believe will tear any object that enters into pieces. In truth, fractal cores are a form of inter-dimensional wormhole, providing two-way passage between the Light Zone and the Dark Zone.
It is unknown whether an opening in the Light Zone provides a tunnel into the Dark Zone at a precisely corresponding point, or simply between any two suitable fractal cores, as the universes likely differ enough that it is difficult to compare locations.
Members of the Insect Civilisation appear to be incapable of passing through fractal cores, likely owing to their entirely organic composition. The Lexx however is clearly shown to be able to pass through, likely on account of being part mechanical, and thereby possessing a metal super-structure along with other standard ship's systems that an insect would otherwise lack.
A fractal core is first encountered in Lexx 1.1 "I Worship His Shadow" when the Lexx and its crew escape the cluster by recklessly fleeing through a fractal core with the Megashadow in pursuit, successfully entering the Dark Zone.
The crew of the Lexx later pass back through a fractal core during Lexx 1.4 "Giga Shadow" into the Light Zone in search of a new source of protoblood for Kai, and subsequently return to the Dark Zone through the same fractal core in order to escape the Gigashadow. It is stated that at the end of Giga Shadow, the fractal core was destroyed and that the crew is now stranded in the Dark Zone.
Later, the crew return to the Light Zone through a "black star" in Lexx 2.1 "Mantrid", after being tricked into believing Kai's protoblood was once again running out.
Finally, the crew passes back into the Dark Zone in Lexx 2.20 "End of the Universe" when Mantrid Drones, having consumed the entire mass of the Light Zone, converge upon the Lexx's location, causing the universe to collapse, but forming a temporary fractal core in the process, catapulting the Lexx back into the Dark Zone.