It's just a theory
Scientists and laypeople use the word "theory" somewhat differently. A theory in science is a way of organizing knowledge. Some theories, like the theory of gravity and the theory of evolution, are based on established fact that those phenomena/systems do exist. There is always more information about them that we'll continue to discover.
Here's a good short explanation: https://www.livescience.com/21491-what-i...heory.html