Transgalactic Stage

The Intergalactic Stage is the fourth and final known stage of a Flood outbreak. This stage is reached once the Flood has consumed all sentient life in an entire galaxy and gained complete understanding of any technological and scientific knowledge that was present. By this time, it has reached a complete critical mass, which means it must depart to other galaxies for further reproduction, infestations and new technologies. Taking into consideration the fact that the Flood is extra-galactic to the Milky Way, the Flood has reached this stage at least once in its existence.

Trivia
This stage was seen by the end of Halo 3