Ok I've been playing on a huge FFA map over the past few days.

Why do the rebels keep attacking all of my planets? My planets have high allegiance but every planet often gets attacked by small rebel fleets with about 10 ships that are a serious annoyance. I hate them worse than pirates.

Its a drain on your resources to keep sending ships to fight the rebels (since they actually have heavy cruisers) and they always attack planets without garrisons with enough force to destroyt structures and hurt my infrastructure.

Where do they come from and how do you stop them?