If string theory is correct, there are a number of higher dimensions, the existence of which would influence gravity at short distances. The idea is that some gravity leaks away to the other dimensions. This is a function of distance, so at short distances less gravity has leaked, and gravity should be stronger. Efforts to find this effect haven't turned anything up yet, though.
Actually just to nitpick, even if we could find one string theory that works, remember that it's "only" math. Meaning that it could totally work to describe a phenomenon without having any physical reality to it.
Even if it predicts observation, like Bohr's model of the atom did, you have some degree of abstraction between the model and the physical implementation, so to speak.
And that's the ultimate problem of string theories: even if we crack some math model, if it can't be observed directly, we just don't know. Quantum mechanics currently sit in-between the purely theoretical (where we do have a model that works, minus gravity, if applicable at that level) and the barely observable (we can infer, a lot, but not much in the way of actually seeing things; although I think nowadays we've crossed that bridge anecdotally enough to be confident but don't quote me on that).
Therefore, string theory models may require higher dimensions in the math to hack the problem, but it doesn't mean these actually exist — like Clifford's algebra, Geometric Algebra, you reason with 4-dimensions to treat 2D (x, y) problems, and 8-dimensions to treat 3D problems — but that's just the math, and the beauty of that is indeed to "unify" other algebras which become essentially subsets.
It's really important to keep that in mind when "gunning for" string theories, or seeing them as "explicative", interpretable. They're really not, at least not now, and possibly ever due to their construction. In truth, it's more about brute-forcing the issue (kinda like current deep learning?), hence the wild ranges of "possible solutions", from 11 to 26 dimensions (some go in the hundreds in suggestion) because they're basically trying to use computers to identify dimensions wherein our universe "fits" quite well, and derive what we don't know (e.g. quantum gravity, or whatever that means in the "promised dimension level"). At a "basic principle" level, that's what string theories are after. It's pure math, it's crazy smart, it's really elegant and almost godly, noble because it requires the state of the art of our evolution and technology; but like quantum mechanics the "interpretation" will not be delivered with the equation, and it may never mean anything to us.
Iirc, LIGO confirmed that there was no leakage from/to other dimensions, hence that we can safely reason (at least on this problem) within our current 4D manifold.
It doesn't (cannot, really) state about the possibility that our universe, though self-contained in 4D, might be floating in a higher-dimensional space, like a 2D surface would float in 3D (and could be internally very consistent).
String theory, by the way, might not prove or disprove any of that (see my other reply to parent).
