Well...not quite a fact. It's a widely accepted scientific theory that probably is true, but it hasn'tand can neverbe proven, much in the same way it's not proven that matter cannot go past the speed of light, or that time travel is impossible. It will always be called into question by science-deniersusually the same people who deny global warming, which has been proven, time and time again, and can even be proven with simple logic. If such an obvious thing can be prevented from being accepted as fact, something that is impossible to prove will definitely be called an "opinion" by so-called "experts."