A scientific theory is supported by evidence, can evolve over time, and is true all the time.
Scientific theories are NOT guesses but rather they are statements that are PROVEN correct through research, examination, and testing. Scientific theories can evolve as well, like Newton's laws change, and they are true all the time because they are called theories, theories are FACTS, definition of fact means true statement, not an opinion but rather fact, only hypothesis are not true all the time and opinions and guesses and etc that haven't been tested and confirmed.