The higher-dimension space within which the brane which defines our universe is embedded. The existence of the bulk is entirely theoretical, although there are indications that the highest Archailects have experimentally confirmed its existence, and may even use its properties on a practical level.
All fundamental forces in the known universe (with the exception of gravity) are confined to our own world-volume, which is a brane consisting of three dimensions plus time. The bulk has at least one extra dimension compared to our universe/brane, which would allow additional universe/branes to exist in parallel next to our own. Because graviton particles can be modelled as an open string, they could conceivably pass between these locations and affect nearby branes. An unknown number of attempts to signal nearby branes appear to have been made by different archai at various times; most of these attempts are believed to use gravity waves, and on occasion apparent out-going signals of this kind have been detected by nearby modosophont observers. However there is no indication yet that such attempts to communicate have been successful.
Most cosmological theories suggest that the bulk itself cannot support matter or processing directly, but instead it is a space that can support lesser world-volumes which may support complex entities including life. Among archailectologists there is a widespread belief that the highest archailects can extend their processing substrates into the bulk, not directly but through the creation of basement universes connected to our own.
Nevertheless there are a number of persistent myths and rumours that the bulk holds wandering demons that can attack universes from without, or at the very least advanced civilisations capable of processing in multiple dimensions.