Anyways, I got to thinking that in the bungie universe, big shot aliens with really big ships might want to do some gateway traveling. Now when I say big, I mean really damn big huge ships. It's not totally unlikely that it may be some sort of dormant gateway. If you don't think so, consider for a minute that the halo really does simply house some kind of ancient super weapon. The more one thinks about it, the more sense it begins to make.
Lets add the builders of the halo into the picture.
Super advanced, and really old, this space farring civilization enjoys traveling to nearby galaxies from time to time. The covenant chased the humans to the ring because they were afraid of them activating or unlocking or calling upon the gateway. The other species in the area are all well aware of the gateway, and they probabaly have some sort of pact between themselves to stay away from it. That would explain its empty state when we arive, but it gives a great reason for the Covenant to chase us. Also, it puts in place a nice excuse to have old stuff lying around for us to find, and possibly allows for really cool suspensefull ending.