The front half of the ship made it through the portal back to Earth. The back half, the side Chief and Cortana were stranded on, was cut off, but still made it through the portal. They simply didn't travel all the way back to Earth. Because of the Ark's location, it has no background stars to be near. If the ship were even remotely close to the Ark, these stars we are looking at wouldn't be there. And they most certainly are stars as they are surrounded by nebulous gas.
Because of the presence of stars, this shows that the ship did travel back within the galactic disc. However, it did not make it all the way back to Earth. This makes me wonder if a straight line distance from Earth through the Pleiades cluster and out a few hundred thousand light years would send us back to the Ark's location outside of our Galaxy.
For additional information about the Pleiades Cluster, click here.
EDIT: I just found another interesting source of information located here. It talks about the distances of Human-controlled space.