The difference between theory and simulation is that theory is the set of equations that explains a phenomena. After an observation of nature is made, theory is established to be able to calculate and predict further phenomena in relation. However simulation is built off of this theory- it only exists as a base of using existing formulas to create new scenarios that can be tested. This seems like a fine line to draw, but the distinction is important. Theory is creating new equations, and simulation is creating new situations. While both push forward knowledge of unknown phenomena in a lot of cases, they are still distinctly separate from each other.