Dylan Cole, an artist formerly with Ensemble Studios, has on his personal Web site some concept pieces for a canceled Halo project that may or may not have been called "Halo Universe."
Ensemble of course was the muscle behind a Halo MMO that was greenlighted and then scrapped sometime last year. Cole's pictures (two are on his site) are titled "Halo Universe City", and in his description, Cole capitalises "Halo Universe" a second time. Could have been a working title of the game. Joystiq also points out it could have been "Universe City". Which itself could have been a placeholder name. Who knows.
Of course it all doesn't matter. The project is kaput. But the pictures are pretty, and tell us a little more about an evolution of this franchise not meant to be. For now.