The structure of the cosmos has always been a subject of intense debate, with new discoveries increasing the complexity of cosmological models every few centuries or so. By 100,000 CE Terran civilisation had formulated the Unified Model, which describes the Byzantine structure of existence as we know it.
3+1 Spacetime ("Normal Space")
The regular three dimensions of space plus one of time that most Terrans are familiar with. However, no curvature has been found in the 3+1 spacetime - it is highly flat indeed, indicating that it is infinite in spatial extent. Combined with the ergodic matter distribution - meaning that if you imagine generating an ensemble of universes, each with its own random initial conditions, then the probability distribution of outcomes in a given volume is identical to the distribution that you get by sampling different volumes in a single universe. In other words, it means that everything that could in principle have happened here did in fact happen somewhere else - this means that there is almost certainly a parallel Hubble volume to ours, some unimaginable distance out there. Various forms of matter exist in this space:
Light Baryonic Matter
This is the universe of stars, planets and galaxies that can be seen from any clear night sky.
Dark Baryonic Matter
This is the vast ocean upon which Light Baryonic Matter floats like froth. Brown dwarfs, very dim red dwarf stars, and cold stellar remnants such as grey dwarfs and black dwarfs make up the majority of matter in this category. Due to the extreme age of some of the grey dwarfs and all of the black dwarfs so far discovered, it is speculated that the DBM universe somehow represents the remnants of an earlier universe.
Weakly-Interacting Massive Particles/Supersymmetric Matter
Also known as "true dark matter", these are the counterparts to normal matter as predicted by supersymmetric physics. Found naturally in the hearts of extremely massive stars, in the accretion disks of black holes, and other regions of the universe with extremely high temperatures and/or pressures. Outside of these regions, supersymmetric matter will decay into neutrinos under natural conditions, although there are artificial means of storing, transporting and using supersymmetric matter for various technological purposes.
This is matter with a predominantly "right-handed" symmetry, as opposed to "normal" matter which has a predominantly left-handed symmetry. This mirror matter interacts with ordinary matter through gravitational interactions and the "oscillation" of neutrinos into their mirror matter counterparts. Every spiral galaxy is accompanied by a spherical halo of mirror matter, and elliptical galaxies have been found to have spiral structures of mirror matter within them. In other words, in the mirror matter universe, a spiral galaxy is an elliptical galaxy and vice versa.
In order to keep the sums balanced, the Unified Model requires the existence of a time-symmetric universe. However, observations of cosmological expansion indicate that it is accelerating, thus ruling out a time when the universe will eventually stop expanding and start re-collapsing, with time reversing itself. But the discovery of tachyons showed that every particle of non-imaginary rest mass has a tachyonic partner of imaginary rest mass. Tachyons travel backwards in time relative to their non-tachyonic counterparts, thus providing the temporal symmetry that the Unified Model requires.
The existence of negative matter and energy, as well as having numerous technological applications, also provides the necessary balance to bring the mass-energy of the universe to zero. Although the concept of an object that masses less than nothing may seem absurd at first, it makes perfect sense when considers mass as a curvature of spacetime, and realise that curvatures can be concave (positive) or convex (negative).
The Cosmic Horizon
The slowness of light compared to the vastness of the universe affects how far we can see, since the light takes so long to get from there to here. This means our Milky Way galaxy sits in the middle of a volume beyond which there appears to be nothing, but if one were to travel to this apparent edge of the universe, then it would look pretty much the same as it did in the Milky Way.
Post-Inflation Era Bubbles
During the inflationary period after the Big Bang, the rapid expansion was in a chaotic state that led to the creations of bubbles, volumes where the expansion was going at a less breakneck pace. Although they are too far away to travel to directly even with FTL technology, the existence of these bubbles has been confirmed through the discovery of entangled wormholes, which formed before the inflationary period and which subsequently have ended with their ends in different bubbles. These wormholes exist naturally in a collapsed state, requiring large amounts of negative mass/energy to prop open in order to explore the other side.
When the Big Bang occurred, the 22 dimensions aside from the ones we are familiar with collapsed until they were smaller than quarks, those dimensions being the different levels of hyperspace that we know of today. Each of them are accessible using Hypergate and Hyperdrive technology, which focus sub-stellar levels of positive and negative energy into small volumes of space in order to create openings into one of the levels of hyperspace.
In the quest to investigate beyond the Big Bang, it was found that our collection of 3+1 dimensions plus the 22 levels of Hyperspace formed but a single "Plane" out of many others residing in a greater volume which came to be called Kataspace. Over the course of 1e1000 years or so, these Planes can drift and end up passing through each other, creating in both intersecting Planes a massive burst of positive and negative mass/energy, the long known of Big Bang.
Threadspace is understood to constitute the "workings" of material reality, with fundamental particles being understood as one-dimensional structures extended throughout a static spacetime. Some have compared it to an operating system that is written entirely in spaghetti code, but that would be like comparing genomes with recipes. It appears that each type of mental architecture has a unique method for manipulating these threads using thought alone, the basis for psyonics. It should be pointed out that doing the mathematics required for modelling the effects of sapient thought on the universe would give even most Artificial Intelligences a difficult challenge.
A collapsed 0-dimensional space which permeates the cosmos. This was what was found to be responsible for the quantum entanglement effect that allows Stargates, Ansibles and Powercasters to operate.
The discovery of this aspect of reality was a pure accident - an AI near the Orion Nebula was conducting experiments into the fermionic resonance of electrons, when the observer probe Curious Stigma suddenly disappeared. It transpired that the probe had been shunted into a parallel timeline, strangely enough one where she had decided not to venture through that region of space. Luckily Curious Stigma managed to recreate the conditions so as to bring the news back into our timeline.
This is a relatively new branch of enquiry upon which the Unified Model of cosmology is partially based. This discipline straddles mathematics and physics, and attempts to explain the unreasonable effectiveness of mathematics in physical inquiry. Metamathematics suggests the existence of other realities with different structures that are modeled using completely different sets of equations.