I apologise if someone has already pointed out this out to the professor and you have moved on:
http://simple.wikipedia.org/wiki/Scientific_theory
In everyday speech, the word "theory" is used as a "best guess". In modern science, a scientific theory is a tested and expanded hypothesis that explains many experiments and fits ideas together in a framework. If anyone finds a case where all or part of a scientific theory is false, then that theory is either changed or thrown out.
http://www.fsteiger.com/theory.html