From Halopedia, the Halo wiki
The Transgalactic 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.
During the human-Forerunner wars, the Flood was driven outside of the galaxy. Doing this may have caused them to achieve the Transgalactic Stage as they were able to return to the Milky Way 10,000 years later when they made first contact with the Forerunners on Seaward, sparking the Forerunner-Flood war. This is likely, as prior to the Halo Array's firing, Forthencho's essence was sent down to Earth by the Gravemind to tell the Librarian what he was told by it. Among this information was that the primary Gravemind had consumed at least ten thousand planets and many galaxies, reinforcing the possibility that the Flood has at one point in history reached this stage.
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