What determines the validity of a theory?

In science, a theorem is an explanation of a phenomenon that has a significant body of evidence supporting it. Theorems start life as a hypothesis for why something happens the way it does. Scientists design experiments to test that hypothesis, and if the results support that hypothesis, it evolves into a scientific theory. According to the National Academy of Science, the best way to evaluate a theory is to determine if it is comprehensive, tends to be well-established, has ample evidence to support it, and is unlikely to change with new data.
One of the most common places to find the scientific communities definition of a theory ironically seems to be when people discuss evolution. Evolution is a scientific theory, and the evidence in support of the theory has been demonstrated and observed time and time again. It is unlikely that there will be observations not consistent with it in the future, and at this point, it is a widely accepted explanation for how life changed over time.
Keep in mind, though, that "unlikely to change over time" does not mean "will not change over time." With new data and better technology, we often have to modify previous ideas for how things work. Sometimes we have to change things significantly, such as with the advent of quantum physics or relativity.
https://www.nas.edu/evolution/TheoryOrFact.html

https://www.space.com/17661-theory-general-relativity.html