Who says the universe has bounds? The classic analogy is of an inflating balloon. If you make two marks on the balloon, they'll expand away from each other, but neither one is the center.
Of course AFAIK there isn't any solid proof for the "finite, unbounded" idea, mostly just speculation. But it was Einstein's thing so we (laymen ;) take it pretty seriously.
Well, Einstein is ;) But yeah, for the sake of the analogy we're talking about a 2-d universe which is the surface of the balloon, nothing else exists.
Of course AFAIK there isn't any solid proof for the "finite, unbounded" idea, mostly just speculation. But it was Einstein's thing so we (laymen ;) take it pretty seriously.