In string theory one studies strings moving in a fixed classical spacetime. ...what we call a background-dependent approach. ...One of the fundamenta… - Lee Smolin

One of the evident facts about the world is the stability of empty space-time. In classical general relativity we can explain this as a consequence of the positive energy theorem... the positive energy theorem must extend in some suitable form to any viable quantum theory of gravity.

Many of us believed in the possibility of a principled explanation for the laws of nature. We hoped to discover a short list of principles, which could be realized in a unique theory, which would retrodict the standard model and uniquely predict the physics to be discovered beyond it. The shocking implication of the results of Strominger reported in 1986 was that it was not to be, at least within the confines of string theory. ...String theory offered more, however... It offered the promise of a setting in which the different perturbative string theories are realized as expansions around solutions of a still more fundamental theory. ...That more fundamental theory would have to be background independent...

There is no fixed, eternal frame to the universe to define what may or may not exist.