Have any of you been to the green barrier? On any level if you warp in any direction and you hold it, you will come to a green barrier that you can't pass through it and and no weapon can penatrate it.
That's the edge of the universe. Seriously. Check out the hera documentation, and one of the flags in the object attribute panels is the Bounces when hits edge or something like that.
This is the limit of the level - you aren't going to bash into it unless you're trying. It's simply a 'ceiling' to make sure that Ares doesn't calculate objects forever. For example, the asteroid belts are really an assortment of objects that spawn asteroids in certain directions. Asteroids have to constantly be created so the belt looks continuous, but unless the asteroids disappeared eventually, Ares would quickly have too many objects to calculate. Thus the ceiling.
Don't worry about it.