In scientific reasoning, a hypothesis is an assumption made before any research has been completed for the sake of testing. A theory on the other hand is a principle set to explain phenomena already supported by data. Theories will pull together experimental results to provide full explanations such as "The Big Bang Theory." Outside of scientific reasoning, "theory" and "hypothesis" are often used interchangeably, and "theory' can unfortunately be interpreted to mean "less sound" or "lightly speculated."
