The Pa'anuri as they appear on a gravyscope
The Andromedan dark-matter entities known as the Pa'anuri are the reason for much of the galaxy's history. They were first encountered many eons ago by the F'sherl-Ganni, the first known inventors of the teraport. The effects of teraporting are injurious to them, and for a very long time they dealt with this by destroying the solar systems of any civilization that invented them. The F'sherl-Ganni were the first civilization to survive finding out about them. The result was a long and bitter war between the two overlaid universes, which eventually ended in a treaty. The Pa'anuri retreated to the Andromeda Galaxy, and the F'sherl-Ganni set up the wormgate network and initiated the infamous copy-and-interrogate policy in order to find and suppress independent invention of the teraport.
Dark matter does not interact with baryonic matter except gravitationally, which means that the Pa'anuri effectively live in a parallel universe. However, the two aspects can communicate via gravity waves, and also attack each other with gravy. Individual Pa'anuri (they are their own warships) are too large to be significantly harmed by gravy weaponry at our scale, but the tiny wormholes created by teraports can harm them, and they can even be killed by very rapid teraporting or high-density teraport-denial fields.
About 100,000 years ago, a wormgate sent by the Pa'anuri from Andromeda arrived, its purpose being to deliver to the F'sherl-Ganni the plans for a galactic drive engine intended to prevent the Milky Way's eventual merger with Andromeda. The device was completed during the Teraport Wars, but it was a trap; when activated, the "engine" caused the Milky way to pinch itself off as its own little universe, and then began destroying the galaxy from the core out.
It might have been possible to repurpose the wormgates and reverse the anomaly created by the bogus drive, but by the time the anomaly was discovered it was too late to do so. However, the bizarre physics of the anomaly allowed Captain Kevyn Andreyasn to use the Andromeda wormgate as a time machine, taking him back to before the "engine" was activated. Once he had done this, he managed to convince the Fleetmind of the danger, and the Fleetmind in turn gathered a coalition fleet and headed to the Core.
Kevyn and the Fleetmind managed to convince the F'sherl-Ganni not to activate the "engine", but the Pa'anuri who were observing began to attack. This so-called Core War took a heavy toll in the fleet, but it bought time to create a giant teraport-denial field that killed every Pa'anuri in the galaxy. The Fleetmind promptly stole the entire coalition fleet, rebuilt the "engine" into a power source, and took the war to Andromeda. The war there is still raging.
The word Pa'anuri (or possibly Paan'uri) comes from a F'sherl-Ganni phrase meaning "That gravitic anomaly is giving me the willies". The Pa'anuri have their own language, in which they call themselves US and beings made of baryonic matter ANNOYING.