If you look up the definition of "theory", you will get a definition that describes an explanation of something that has been tested many times and generally is accepted to be true.  I taught my students for years that a theory is someones explanation for how something works, or why something is the way it is, as opposed to a "law"  which has been tested so many times, it is generally regarded as being true.  Before Sir Isaac Newton, no one could explain gravity, they just knew things fell towards the Earth.  Newton, a very gifted scientist and mathematician, was the first to think of some type of force that drew objects together.  Einstein was the first to "think outside the box" about motion being relative to the viewers perspective, and also throw in the factor of the viewers motion as well.  This radical notion superceded Newtons laws of motion that had existed some 200 years.  Charles Darwin was trying to come up with a rational explanation for the many varieties of wildlife he observed on the Galapagos Islands.  Why were there so many different type of beaks on finches, for example?  But the truly common thread that ties these three titans of the science world together is the fact they were willing to go outside the parameters of the established world of scientific knowledge that existed at that time.