The word theory here describes an interconnected web of observations, deductions, and predictions. In the latter sense, theory is equivalent to hypothesis . One may propose several, co-equal competing hypotheses for a specific cause or effect. Such direct alternatives rarely exist for scientific theories such as evolution, plate tectonics, or quantum physics. For that reason, this use of the word theory is quite different from its typical or colloquial use.
All scientific theories draw upon observational evidence.
They also entail deductions and predictions , often made by applying mathematical models to interpolate or extrapolate from observational data. The only rationale for retaining a particular scientific theory is the success of the deductions or predictions one makes using the theory. The scientific method does not allow us to prove any individual hypothesis correct. The scientific method only shows that its alternatives are incorrect or less correct. Any scientific theory must then have observational consequences that could conceivably show it to be incorrect.