When scientists use the word theory, it has a different meaning than normal everyday use. In science, a theory is not a guess or a hunch.
It is a well-supported, well-documented, and well-substantiated explanation for observations.
For example, the theory of gravity does not mean it is just a guess. It has been tested and all observations support it. You can observe it for yourself, too. Theories in science are never proven. They stand until something proves them wrong. Evolution has been tested in thousands of experiments and has stood the test of time ever since Darwin presented it over one-hundred and fifty years ago. Advances in physics, molecular biology, geology, genetics and chemistry have refined, supported and expanded evolutionary concepts beyond anything Darwin could have imagined.