Colonisation of the Orion Arm by the Terragen Civilisation originated from a single location, Earth, and spread out in a number of waves of increasing intensity. The first wave was that of the a-human AIs, many of which colonised minor systems in the Inner Sphere and elsewhere which were unsuitable for humans, and generally these systems became isolated no-go zones.
Some of these solipsist AI clades that were not assimilated by the second wave front, that of the main terragen empires, did simply remain in pockets and no-go zones. Rather they cleverly turned things around and decided to do the assimilating themselves. A number of aggressive AI factions overwhelmed or subverted nearby human colonies, and threatened to expand back towards the Inner Sphere. Backscattering waves of a-human AI have been a fairly common occurrence throughout the history of the Terragen expansion.
Similarly a number of human colonies began to send out secondary waves of colonisation themselves; Penglai and Beta Virginis were two early examples, while the Conver Ambi became expansive and almost overwhelmed the Inner Sphere at one point. More recently the Oracle Machines and the Laughter Hegemony have produced strong secondary waves of colonisation, leading to serious interstellar conflicts.
Some evidence of backscattering waves has been observed in distant high-energy emission civilisations, in the Milky Way galaxy and elsewhere, sometimes apparently leading to the disappearance of whole cultures. It is possible that some, or many, of the vanished civilisations of the past also disappeared because of backscatter of this kind, although the importance of this effect is a matter of debate.