The question as stated demonstrates the public's confusion over the use of the term "theory." To many people in the general public, the word "theory" means an unproven statement or an educated guess. Scientists would call this a "hypothesis." Theory, on the other hand, is what scientists pray for. It is a construct that is not guessed, but a formula that organizes existing, observable phenomena, has the ability to integrate new observed phenomena in a way that makes sense of it, and presents hypotheses that can be shown as either accurate or inaccurate over time. The "theory of gravity" is a good example. It is not called that because gravity is an unproven hypothesis, but because it creates a construct that fits the data generated by physics, mathematics, astronomy and other scientific disciplines. The theory of evolution is likewise not a guess or even a hypothesis. It is a construct that fits together and makes sense of data from a variety of inter-related fields in life sciences, including botany, zoology, medicine, genetic research, paleontology, and so forth. "Theory" is not the starting point, it is the highest order of scientific organization of data. Scientists do not lightly give the term theory to anything that does not organize massive amounts of data. The public use of the term has created huge confusion because it comes very close to being the exact opposite of what scientists mean.
The definition of a scientific law is the description of an observed phenomenon. A theory gives an explaination of said law. A theory will never become a law and a law was never a theory. Any scientific law has always been a law whether we knew it or not. Gravity was still there when we were "cavemen". The phenomenon of evolution was going on before there was a theory to explain it, and it continues to go on today. The reason that many scientists accept the theory of evolution as fact is because scientifically, massive evidence suggests that it is very true.