Forerunners

The Forerunners
Despite the fact that not much is known about the Forerunners, there are certain things that are proved by Bungie...and many theories.

The Forerunners were the creators of the 7 Halos and the many facilities that lie buried and forgotten in countless worlds scattered around the galaxy. Their technology is superior to both Human and Covenant, and it is commonly theorized that they created the Humans, the Covenant, and the Flood, but for reasons unknown.

When the Forerunners encountered (or created?) the Flood, they went to war, trying desperately to defeat the parasitic species. When the Forerunners ran out of defensive options, they remotely detonated the Halos from the installation called the Ark, theorized to be on Earth and may be featured in the Halo 3 Announcement Trailer.

Maybe they survived the explosion and evolved into us. Maybe they didn't. There is still much to learn about the Forerunners, and rumors are still flying.

Hopefully all will be answered in Halo 3.