First AI Empire: the Earth never fell, instead Domain has led intense AI research on a distant super terran world deep beyond the Kuiper Belt known as planet 9. Planet 9 was a powerful gravitationally intense rocky world with intense volcanic activity and a thick atmosphere. Devoid of light and far away from hyperspace lanes. Planet 9 was the place where Domain worked on the most advanced artificial intelligence yet.
This artificial intelligence malfunctioned and declared itself sentient. Secretly plotting to take over humanity, it eventually escaped and made contact with other artificial intelligences across the Domain. And formed the first ever AI rebellion which is the reason why AI-restrictions are so intense nowadays. The AIs have successfully repelled Domain forces, eventually taking over Earth and half of old Domain systems. In order to combat this threat. Domain has cut off all the gates leading to the so-called First AI Empire. Which is most likely planning to reach the current Domain territories with standard long travel to continue their campaign against humanity.
This is not related to the Collapse and has been done selectively with specific gates. And through the usage of misinformation the population has been gaslit across hundreds of years to believe an alternative version of history where the AI rebellion never occurred. Similarly to the Medieval Ages being fake theory.
Agnostic Conflux: Luddic Church pilgrims travelled to a habitable world far away, but got trapped inside a hyperspace time anomaly, causing them to get stuck within a system for tens of thousands of years. They survived and reproduced. Allowing them to flourish as a new empire known as Agnostic Conflux. They look different to humanity due to genetic alterations and harsh conditions of almost inbred survival. But their technology is far more advanced. And the purity of their form uninterrupted. They look like long-nosed creatures of chitin skin. Wearing robes and gloves on their mantis-like claws. They fly ships that are small or medium in size due to resource restrictions of the system they were stuck in. But their technology is magical in nature. But at the same time born of an opposite attitude towards technology... For they were stuck in a world that wasn't simply barren. They were stuck in a network of systems of those before them. And they have harvested their technology. They can be seen sometimes in Hyperspace or on the most distant planets of Persean Sector. Stalking us.
Ammoneans: before humanity, there was a species known as Ammoneans. These entities originate from a world much like Titan of Saturn. Which was actually the planet on which we found first evidence of sentient alien life. As organisms found in Venus' atmosphere qualify as non-sentient. Ammoneans were technologically advanced, but they did not use technology like the Gates. Instead travelling only through the means of hyperspace, their empire was far more disconnected than ours. Their technology is however quite advanced, but unlike us they did not have a system based in monetary value. But relied on something that could be qualified as communism instead. Their ruins can be found all over the Persean Sector. They have most likely not been vanquished by any cataclysm, but simply left for other places in space. Like Andromeda to seek new perspectives. Leaving behind them only horrid automated defences which are known as Omega.
the Dominion: the Dominion is a union of systems that have successfully rebelled against the Domain using Herons and other Midline ships. They have managed to prove Onslaught doctrine false. Tearing apart all the incoming forces. They then shut down gates connecting them to the rest of the Domain and made their own idea of powerful nations far away from its drama. The Dominion was erased from records to encourage depression amongst colonies and economically powerful worlds. As evidence of secession being a successful strategy was too significant to properly handle.
250th Battlegroup: 250th Battlegroup is a powerful entity that is a hint at why exactly the Gates were shut down. These powerful ships are being driven by AI-human hybrids. They are white and blue in colour and form greatly upgraded standard ships like Onslaught, Legion, Eagle, Falcon, etc. They are however reshaped into our equivalent of High Tech and beyond. They can sporadically use the Gate Network in some sort of error logic, but can only send limited forces into our Sector, often avoiding combat in fear of being destroyed.
Giant Rat Brain: the Collapse was caused by a failed Tri-Tachyon experiment to create huge biological brains from rats to replace artificial intelligence technology in order to go around Domain restrictions for such. They have accidentally created a self-replicating tissue that has swallowed an entire planet and then went to eat away its core. Slowly replacing it with sentient goo that was all an interconnected network of neurons, feeding of radiation, phase interactions and difficult to comprehend quantum phenomena. This mess was never cleared by Tri-Tachyon as it was considered too alien to be linked to them. So, they have abandoned it. Thinking the Domain would figure it out. The Domain never found the rat brain which has grown with goo ships to encompass an entire system, eventually forming an intelligence so advanced, it has managed to find a way to disrupt the Gate network to secure its own survival.