alt Hacker NewsBlack holes were theorized and you could design experiments that, given the proper instruments, would allow them to be detected. Relativity was similar (famously, the curvature of spacetime was demonstrated during a solar eclipse by being able to see stars that should have been behind the sun).
String theory has nothing even theorized that would allow us to prove it.
String theory, being a high-energy theory, predicts that at high energies things will look “stringy.” It’s not untestable in principle. But it is out of reach of current experiments. I think that distinction is important.