A theory is a unifying explanation for a broad range of observations. Science has come up with the scientific method to answer questions through observation. When something has been repeatedly experimented and results are yielded from them scientists cannot conclude answers to their problems just yet.
A controlled experiment needs to be tested multiple times until it yields an average solution that would generally be coming from different testers. Basically, a theory is something that has been repeatedly confirmed through some form of experimentation and observation.