The current version of string theory is called M-theory, introduced in 1995, which is a comprehensive theory that includes the five supersymmetric string theories. M-Theory exists in 11 dimensions. There’s just one problem. No one knows what M-theory is.
Scientists are searching for a complete string theory, but they don’t have one yet. And, until they do, there’s no way of knowing that they’ll be successful. Until string theorists have a complete theory that describes our own universe, the theory could all be smoke and mirrors.
Although some aspects of string theory may be shown to be true, it may be that these are only approximations of some more fundamental theory — or it may be that string theory is actually that fundamental theory itself.
String theory, the driving force of 21st-century theoretical physics, could prove to be nothing more than a mathematical illusion that provides some approximate insights into science but isn’t actually the theory that drives the forces of nature.
It’s unclear how long the search for a theory can last without some specific breakthrough. There’s a sense (among some) that the most brilliant physicists on the planet have been spinning their wheels for decades, with only a handful of significant insights, and even those discoveries don’t seem to lead anywhere specific.