In science, a theory needs a body of evidence to support it. A theory in science explains some observations in the material universe, and these include explanations and predictions that can be tested. If you cannot test these explanations or predictions, or they are not falsifiable, then it isn't a theory of science. Usually a theory is based on a long term body of work built over time on experimental observation and repeated testing by various people (if others cannot repeat your experiments using the same conditions and methods, then the theory also fails). Faith does not come into it, as it is all about what can be shown, measured, and repeated, not what is believed. I wouldn't completely agree with that, personally. While there is definitely a strong basis of evidence involved in science, there will also always be a level of faith involved in believing the theory to be correct. That faith may not be blind, but it still exists. Take for example the theory that life has existed on Mars at one point or another. Multiple studies have been done to test if life is possible, which is centered around three ideas: the presence of heat, the presence of organics, and the presence of water, water being the most crucial. Scientists at this point have found two out of those three, the third being organics. As such, many scientists believe life has existed there. Do they have 100% definite proof of this? No. Will they ever? Who knows. And if they do find organics, will that give them definite proof? Of course not; the only way they would have 100% proof is if something living actually came into sight and proved itself to be living. And yet many scientists hold strongly to the idea of life on Mars simply from sources of water because they have faith. A closer example to home would be gravity. In short, there is no defined source of gravity. No one knows how or why gravity exists. And yet gravity is an accepted idea in every day life. Why? Because when you jump up in the air, you come down again. But jumping up and coming down isn't definite proof that gravity exists, it's only an effect that means something is keeping us from floating away. And while we have no definite proof that gravity exists, we believe that it does, because we have seen results that match up with what gravity is supposed to do.