As your systems grow, they project an influence radius. Uncolonized systems within your influence can only be colonized by you, and there are techs that allow you to slowly convert systems colonized by other empires that fall within your influence.
What factors affect your influence radius? I’m pretty sure population and system level are part of it, but it seems like there must be something else because I’ve seen AI empires with absolutely enormous influence areas when I know they were less developed than me.
Answer
Source: http://cdn.akamai.steamstatic.com/steam/apps/392110/manuals/User’s_Manual_-_Endless_Space_2.pdf
Influence: used as a currency for diplomatic negotiations and to expand the frontiers and the
influence of your empire.
Basically try to generate more influence resource from technologies that grant influence building.
From what you are describing sound like the AI went heavy on influence despite being less developed than you.
Attribution
Source : Link , Question Author : Oblivious Sage , Answer Author : Vyndicu
