What makes a hypothesis become a theory?

A hypothesis is used to make predictions. Experiments are carried out to test these predictions. If the outcome of the experiment was not as predicted then the hypothesis is falsified. It is either rejected or modified. If the outcome of the experiment confirms the prediction then that provides some evidence that the hypothesis is true.

Over time, after testing different predictions, there will be a significant amount of evidence in favour of the hypothesis, and all the main alternatives have been rejected. At that stage the hypothesis becomes a theory.