Age of Abandonment

One of the leading theories is that the Age of Abandonment was the time when the Forerunners left the activated Halo, or, as the Covenant put it, "Embark on the Great Journey". The Covenant believe that the Forerunners underwent the Great Journey, but in fact they disappeared because they activated the seven Halos in order to contain the Flood, thereby killing themselves and all sentient life in the galaxy. This event happened about 100,000 years before 2552, before the rest of the Covenant races had evolved enough to become sentient.