There are two different theories that explain most of the universe. The standard model predicts how the smallest bits of matter behave. Relativity describes large scale structures like the universe, space, and time. The two theories don't go together, and you can't use them together, so we don't know how the whole universe works, leaving mysteries like the big bang, how black holes work, and so on. Some people think that there should be a formula that puts them together, so that relativity has a description on small scale components of gravity and time.
String theory was supposed to bridge the gap and put the two theories together, but it never was able to produce anything, and now many physicists believe that there is no reason that there should be a way to combine them. There is no experimental evidence, or observations, to make anyone believe that there is even a reason to merge those two theories, other than we have a taste for something like that existing.
String theory has tons of beautiful ideas, cool math, and a lot of the smartest people sunk cost in decades trying to make it work, but it hasn't. Worse, string theorists were so effective at publicly advocating for it, and lied to make it sound like it was the next best thing, that it put a lot of people off.
