Many lay (non-scientific) people don’t really understand that a Scientific Theory is actually a serious compilation of scientific facts. When people think about theory they think of the way that non-scientific people incorrectly use the word theory in their every day language. They say things like, “I have a theory about how the stock market responds to hot weather.” What they really mean to say is that they have a hypothesis about the relationship between the stock market and the weather. They can perform an experiment measuring the weather and the market to determine whether their hypothesis is correct. The measurement could determine a statistical relationship between the two, but it is virtually impossible to rule out every other parameter in this relationship.
So, what is a scientific theory if it isn’t a guess about how something behaves? The definition of a scientific theory is: A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena.
In other words, a theory is an explanation of how a series of facts fit together. But, not only that but a theory has predictive power. A theory can be used to create more hypotheses so that further experiments can be performed. Perhaps this is why the public has this misconception about what a theory is. Since you can use a theory to predict completely new hypotheses you always face the point where the theory may hit the wall and fail to be predictive.
An example of this is when the theory of mechanics is used to describe the behavior of objects. However, as the size of the object gets smaller and smaller classical mechanics fails to describe how these small objects behave. So, at the scale of atoms classical mechanics no longer describes how electrons orbit the nucleus of the atom. It doesn’t mean that classical mechanics is just a theory and it is useless. Hardly, instead classical mechanic describes how things which are larger than a few atoms behave. The theory of quantum mechanics must be used to describe the behavior of atoms and molecules.
So, when you hear the fundamentalist Christians tell you that the Theory of Evolution is bunk, because it is only a theory. Then just remember that a scientific theory is a set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena. Remembering this definition makes one realize how powerful the theory of evolution really is.