What is a well tested explanation that unifies a broad range of observations?

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 though it makes bad predictions unless an alternative theory is offered that makes more sense. A theory is an explanation of what and why things happen. A hypothesis is a prediction of what will happen based on a theory.

The Periodic Theory is now frequently regarded as a law because of its exceptional predictive abilities and seemingly constant accuracy.

Despite evidence that sometimes objects gain mass when they burn, the theory that there was a substance in matter that was used up when things burned was accepted for more than a century before being refuted by the availability of the atomic theory and the theory of oxygen.

A scientific theory.