"A scientific theory is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method and repeatedly tested and confirmed through observation and experimentation." Evolution is a theory that explains the facts (such as diverse lifeforms existing and sharing dna etc). A theory isnt an unproven idea it is an explanation of facts, it provides predictions and explanations. In the example of evolution it provides an explanation of how massively diverse lifeforms can share so much genetic similarities and predictions can be made from it (tiktaalik was found by someone working from a prediction based on evolution theory). Gravity is a theory (that explains why when you drop something it falls to the ground etc). If there is no evidence for something then it is not a scientific theory (certainly not a robust one like atomic/gravitational/evolutionary theories) it is a hypothesis. - "In science, a hypothesis is an idea or explanation that you then test through study and experimentation. Outside science, a theory or guess can also be called a hypothesis. A hypothesis is something more than a wild guess but less than a well-established theory."