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.

What is called the Post-Intergalactic Stage is a an unknown theory in which after the Flood supposedly conquer every galaxy possibly habitable, (after trillions of years in time) that they will cause there own doom by realizing the fact in which they depend on life, possibly posing question to the Flood's purpose. Some say it would lead the Flood to their own form of suicide.

Trivia
This is the most likely the stage the Flood was at during the events of the Forerunner-Flood War.