So after I beat the campaign on legendary I went to the Waypoint and watched all those domain videos. What I found was basically a creation story of all life in the universe.
Long time ago, humans were spread about the universe. Some were super advanced and others where simple tribesmen. The advanced ones found the flood and attempted to eliminate all traces of the infection. Some of the flood reached forerunner planets without the forerunners knowing and when the humans tried to kill the flood, the forerunners thought they were attacking them. So the forerunners recognized the humans as the greatest threat to the galaxy and started to fight them (us). They succeeded in doing so then realized they (we) were just wiping out the terrible infection. The Diadect used the composer to wipe out and transform all remaining humans into his Prometheans to fight the flood-a losing battle. The forerunners decided to fire the Halos and completely wipe out all life. Diadect’s wife imprisoned him in Requiem for some greater cause (of which I haven’t figured out), then planted seeds for all life in the universe on various planets. The halos went off and then all life slowly came back.
I am pretty sure this is what everything means so correct me if I am wrong…