I hear this all the time from Christians. Evolution is just a theory or guess. You are using the wrong definition here.
As used in science, a theory is an explanation or model based on observation, experimentation, and reasoning, especially one that has been tested and confirmed as a general principle helping to explain and predict natural phenomena.
Any scientific theory must be based on a careful and rational examination of the facts. A clear distinction needs to be made between facts (things which can be observed and/or measured) and theories (explanations which correlate and interpret the facts.
Now if you said the Bible is just a theory, you would be right, due to not having anything to do with science.
As used in science, a theory is an explanation or model based on observation, experimentation, and reasoning, especially one that has been tested and confirmed as a general principle helping to explain and predict natural phenomena.
Any scientific theory must be based on a careful and rational examination of the facts. A clear distinction needs to be made between facts (things which can be observed and/or measured) and theories (explanations which correlate and interpret the facts.
Now if you said the Bible is just a theory, you would be right, due to not having anything to do with science.