What is a theory?
Scientifically speaking, a theory is a well-tested explanation that has been repeatedly confirmed.
What is considered to be a theory in science is very different than what the media or mainstream western society considers to be a theory.
Theories have been tested extensively and the results or outcomes have been repeatedly been confirmed. They have been confirmed by multiple independent researchers over a long time. They are NOT a best guess or a hypothesis.
Theories unify multiple observations and explain why something has happened.
Evolution, gravity, and general relativity are all scientific theories.