The Ark is 260,000 light years from the galactic center. So the Milky Way we see in halo 3 is from around that long ago.
The halo array fired about 102,500 years ago.
Why didn't humanity stick a Hubble telescope-like thing in orbit around the Ark after the Human-Covenant War to watch the galaxy 150,000 years when it was ruled by precursors? The forerunners didn't rule until around 150,000 BCE, 50,000 years before the Halo Array was used, but I'm sure watching parts of the galaxy ruled by what would become the Flood would be a good idea.External link →