Theory - Definition and More from the Free Merriam-Webster Dictionary
This is how we have theoretical physics. These guys are basing their theories on mathematics in general, but also on pretty much any other thing that might be true just from observations of the universe, then looking for ways to prove the theories correct.
If I said that my theory was that aliens have been visiting Earth for the past 100 million years and messing around with species and life on Earth in general just to see what happens, it would still be a theory. I have no proof and would need to find a way to actually prove it if I ever wanted this to be a widely accepted theory and to get it taught in the classroom. This is why I have that particular choice in the poll. To see if there are some people who would approve of teaching absolutely any theory on the development of life on this planet, rather than ones that we just have proof to support.