You've been told that "evolution is just a theory", a guess, a hunch, and not a fact, not proven. You've been misled. Keep reading, and in less than two minutes from now you'll know that you've been misinformed. We're not going to try and change your mind about evolution. We just want to point out that "it's just a theory" is not a valid argument.
The Theory of Evolution is a theory, but guess what? When scientists use the word theory, it has a different meaning to normal everyday use. That's right, it all comes down to the multiple meanings of the word theory. If you said to a scientist that you didn't believe in evolution because it was "just a theory", they'd probably be a bit puzzled.
In everyday use, theory means a guess or a hunch, something that maybe needs proof. In science, a theory is not a guess, not a hunch. It's a well-substantiated, well-supported, well-documented explanation for our observations. It ties together all the facts about something, providing an explanation that fits all the observations and can be used to make predictions. In science, theory is the ultimate goal, the explanation. It's as close to proven as anything in science can be.
Some people think that in science, you have a theory, and once it's proven, it becomes a law. That's not how it works. In science, we collect facts, or observations, we use laws to describe them, and a theory to explain them. You don't promote a theory to a law by proving it. A theory never becomes a law.
This bears repeating. A theory never becomes a law. In fact, if there was a hierarchy of science, theories would be higher than laws. There is nothing higher, or better, than a theory. Laws describe things, theories explain them. An example will help you to understand this. There's a law of gravity, which is the description of gravity. It basically says that if you let go of something it'll fall. It doesn't say why. Then there's the theory of gravity, which is an attempt to explain why. Actually, Newton's Theory of Gravity did a pretty good job, but Einstein's Theory of Relativity does a better job of explaining it. These explanations are called theories, and will always be theories. They can't be changed into laws, because laws are different things. Laws describe, and theories explain.
Just because it's called a theory of gravity, doesn't mean that it's just a guess. It's been tested. All our observations are supported by it, as well as its predictions that we've tested. Also, gravity is real! You can observe it for yourself. Just because it's real doesn't mean that the explanation is a law. The explanation, in scientific terms, is called a theory.
Evolution is the same. There's the fact of evolution. Evolution (genetic change over generations) happens, just like gravity does. Don't take my word for it. Ask your science teacher, or google it. But that's not the issue we are addressing here. The Theory of Evolution by Natural Selection is our best explanation for the fact of evolution. It has been tested and scrutinised for over 150 years, and is supported by all the relevant observations.
Next time someone tries to tell you that evolution is just a theory, as a way of dismissing it, as if it's just something someone guessed at, remember that they're using the non-scientific meaning of the word. If that person is a teacher, or minister, or some other figure of authority, they should know better. In fact, they probably do, and are trying to mislead you.
Evolution is not just a theory, it's triumphantly a theory!
Theory: 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. American Heritage Dictionary