What is a well tested explanation that unifies a broad range of observations?
A theory is an explanation of what and why things happen.
A hypothesis is a prediction of what will happen based on a theory.
Theories are tested by the hypothesis and predictions based on the theory.
Good theories make good predictions. This is how Theories are tested.
Bad theories make bad predictions. This is how theories are rejected.
Often a bad theory is retained even if it makes bad predictions unless an alternative theory is offered that makes better sense.
The Periodic Theory made excellent predictions. This is why the Periodic Theory is now often considered as a law. It predictions seem to always be true.
The Theory that there was a substance in matter that was used up when things burned was believed for over 100 years despite evidence that sometimes things gains mass when they burned.
It wasn't until the theory of Oxygen and the atomic theory were available that the earlier theory was rejected.