Interstellar Stage

The Interstellar Stage is the third stage of a Flood infestation. Only when the Flood has gained sufficient technology, possess no need for further evolutionary augmentations and have conquered all planets in a given Solar System then they may progress into this setting. Once this stage is reached, the Flood has consumed all sentient life and technology of many planets and with the fleets of their dead enemies, they move on to the next Solar System. After taking an entire galaxy, the Flood begins the Intergalactic Stage.

Trivia
This is the most likely the stage the Flood was at during Halo 3 though the Flood did not infect an entire Solar System yet.