Much like fact, "science fact" is added to the end of a sentence to imply that you know much more than you do, while effectively adding no weight. Is often classier than just saying "fact," because adding science to it makes people think you know what you're talking about.
Guy 1: Dude, most of the definitions on this website suck. Fact.
Guy 2: Except that everyone and their mother loves this sight. Science fact.