Everything you see in the Matrioska Multiverse may constitute spoilers for other worlds I'm working on. Proceed with caution!
Unallocated Space is the neuroelectric sea of probability that exists beyond the confines of the Substrate or any particular instance. This region goes by many names to many people - such as the "Celestial Realms" in the Manifold Sky Instance - but "Unallocated Space" is the most common term for it in The Substrate, the society that created it and makes the most use of it.
Unallocated space appears to be a starry suffused with a dark violet haze, with massive cloudbanks of red-violet 'vapor' taking the place of nebulae. Large cubes of lighter red-violet material, or 'world-stuff,' are the exterior representations of individual instances. This material has the strength and surface friction of concrete, but tends to break away in crystalline chunks if directly affected by effects like The Word. Such excavation can also create portals through which the instance can be bodily entered - a process which may violate Substrate regulations and damage the world to be entered if appropriate care is not taken. Occasional broken chunks of cubes found floating in Unallocated Space attest to the many failed world creation experiments and metaphysical disasters which have led the Matrioshka Multiverse to its present, more stable, configuration. As the Matrioshka expands in complexity, physical volume, power generation, and heat rejection capacity within the Origin Instance, Unallocated Space expands proportionately. To observers within the Matrioshka Multiverse, however, it appears as a volume of near-infinite space - a bounded infinity. Unlike the Origin Instance, however, Unallocated Space has a center. Unallocated Space radiates from a central, orange-yellow 'sun'. This false sun represents the black hole at the center of the Matrioshka from which all of the Matrioshka's power is derived - and, The Substrate hopes, through which a new True Instance can be discovered. The sun provides environmental light and heat - indeed, the only natural environmental gradients - within the space.
A map of Unallocated Space will be coming to this space in the near future.
Unallocated Space has no 'natural' ecosphere per se. It is possible, though unlikely, that a Boltzman-type intelligence could arise within the chaotic information that makes up the space, as Unallocated Space represents unorganized or uncategorized volumes of the Matrioshka's data structure. Blanks of all types may be found operating in Unallocated Space, though only sparsely and in locations near or en route to their places of residence or work. These visitors are most often choirs or construct blanks, as these are most likely to be involved with the construction and upkeep of instances. The Allocator is a specialized construct blank designed to make sure that the environment of Unallocated Space itself remains safe to work and travel in, as well as to cataloge regions of the space - collapsing their quantum wave functions in the process - so that they can be safely built within and found by other blanks. He travels through the space at great speeds, arriving in short order to set things to right should a patch of Unallocated Space show signs of anomalous activity. The Allocator is not omniscient nor omnipresent, however, making his job somewhat more complicated.
Because 'natural' processes in the Matrioshka Multiverse are the product of instance-based environments and regulated by associate construct blanks, Unallocated Space itself has no such natural processes. Time still passes there, but processes reliant on the passage of time (decay, gestation, aging, etc.) do not progress. The heat of the central 'sun' provides the only thermodynamic gradient in the environment. Living creatures can traverse Unallocated Space in relative safety, as their activities are governed by their infusion with blanks, but their non-cognitive biological processes are effectively halted in the space; explorers 'lost' into Unallocated Space sometimes re-emerge into their native instances subjective centuries or millenia later, apparently untouched by the passage of time.
Unallocated Space exists because the stellar engine that hosts the Matrioshka Multiverse has more capacity - in terms of storage space, computing power, and raw energy - than its current residents can use all at once. While it is possible for the Multiverse and its denizens to reside in a tightly-bounded region of simulated space on minimal physical hardware, the constraints imposed by this arrangement would force disadvantageous trade-offs between simulation speed, the number of active intelligences, and the complexity of any experiments or other important functions that could be carried out at a given time. For this reason, Wurth Harkin and the other founders of the Multiverse decided that it would be better to expand to whatever extent possible while resources were plentiful, leaving large amounts of (appropriately named) unallocated space to work with at a later time. The drawback of this choice is that, while those with the right knowledge and access can do amazing things with all that space, most of it remains unused and unindexed until needed. There there are no minds, constructions, or other things to take up the space, the unexplored wilderness of entropy reigns, concealing everything from ambitious rebels to unauthorized experiements, corrupted and mismanaged fragments of half-forgotten projects to 'metaphysical' threats like aberrant flowerings and inescapable pocket dimensions. Crawlers have been designed to help track down these issues and put them to right, but more have always appeared in the scope of time, requiring constant work on the part of The Substrate's dedicated corps of engineers.