Ok, maybe I'm missing something - but are enemy fleets just randomly generated from nothing on the fringe edges of the map? In the unknown regions, I keep having a black sun fleet spawn on Rattatak or Rakata Prime - I control and have controlled every neighboring system for a very, VERY long time and despite that the enemy seems to be capable of spontaneously spawning enemy space and land forces out of nowhere.

Its not hard to put down, but its incredibly annoying.