How a scientific theory differs from a guess or an opinion?

A hypothesis is a guess based on the knowledge of the scientist who makes it about what will happen in certain conditions. An experiment is designed to test the hypothesis, or observations are made in the conditions that the hypothesis describes. If the experiment or observation proves that the hypothesis is wrong, then the hypothesis is modified or discarded. If many, many different experiments and observations, performed many, many times, by many, many different scientists all seem to support the hypothesis, then the hypothesis comes to be accepted by the scientific community as a good working model for the conditions it describes. It then becomes a theory. At any time, an observation could disprove a theory or at least force it to be modified, but all theories currently accepted at any given time have lots of strong scientific evidence supporting them. (It is impossible to PROVE a theory, only to provide evidence in support of it). A theory will alwaysbe reconsidered in the face of new evidence. An opinion may not be. A theory also has much more evidence behind it than a guess.