Quoted from http://www.halopedia.org/Greater_Ark
The greater Ark, also referred to as the Ark or the Beginning Place, was a Forerunner megastructure, and one of the two Arks used to manufacture Halo rings. It was known to have produced the original Halo Array, comprised of rings 30,000 kilometers in diameter as opposed to the 10,000 kilometers of the final Array, which was produced by the smaller Installation 00 instead. Like Installation 00, the greater Ark was located outside the Milky Way galaxy.