args.bungie.org

[spoiler]In one of the levels, you come to a map room similar to the one on Halo that shows a giant map of the instillation. While on the ark, this map first shows the our own galaxy and a small dot. Then the dot expands into the Ark. The Ark is not in our galaxy but outside of it, making it a safe haven from the Halo rings firing. You are able to see our galaxy if you look into the sky while playing on this particular level which may have inspired the "Unknown" image of the Boomerang nebula.

I further speculate that IRIS was so named for obvious reasons. The terminals look like eyes and its through the iris that you are looking.[/spoiler]

I[spoiler]thought it was called IRIS, because of the symbol used.[/spoiler] But that would make sense.