What does it means when scientist develop a theory?

Answer 1

'law' is an observation about how things behave, or tend to behave, without an explanatory framework. i.e., Newton's laws of gravity, Kepler's laws of planetary motion, the laws of thermodynamics. These are essentially factual observations about how the universe behaves.

A hypothesis is a proposition that forms an explanatory framework about the phenomena. It must explain observations, and provide a testable, falsifiable explanation for why the phenomena exist. For example the theory of stellar evolution started out as a hypothesis that explained how stars form, change over time, and how they die, and it provides specific testable predictions for future observations.

A theory is a hypothesis that has gathered supporting evidence, and has been accepted as a valid explanation for phenomena. It's a hypothesis, all grown up. The key is the difference between the use of the word 'theory' in science and common language. 'Theory' does not mean 'guess' or 'supposition' in science. A valid theory is a widely accepted explanation of phenomena. In the example above, the hypothesis of stellar evolution gathered sufficient supporting evidence for it to be considered a valid theory.