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)3 happens, just like gravity does. Don't take my word for it.4 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.5
Evolution is not just a theory, it's triumphantly a theory! ..."
http://www.notjustatheory.com/