The only thing that is not an object (as such) that remains in spacetime are fields, whose state evolves according to the principle of relativity. Thus ensuring

The only thing that is not an object (as such) that remains in spacetime are fields, whose state evolves according to the principle of relativity. Thus ensuring that you cannot measure your position relative to spacetime without getting "you are right here" as a pretty-much useless answer. For what it's worth, the reason we know spacetime exists and is not 'nothing' is because it carries energy around the universe. That's pretty much the main common property shared by things that exist.