Right, so that, er is not what we are talking about. You seem do be under the impression that "facts" are undisputable. They are not. We believe them, because we can put them to every test we can think of, to build a body of evidence that lends credence to a 'theory', hence the requirement for statements to be falsifiable.

This theory of knowledge is a hard requirement for any field associated with research and is well defined. In this case, we must default to the NULL hypothesis ( X does not exist) because we cannot formulate a falsifiable statement for ( X does exist). We can falsify our NULL by providing evidence for X ( X is this, X is here, X can do this). However, for most tests we default to alpha = 0.05 for statistical significance because of convention, and because data must be gathered in batches, sometimes inaccurate. Likewise, proving life on Mars is hard, because we must first falsify every other possible theory before we can claim "the presence of this compound cannot be explained by any other theory aside from the presence of life" as we cannot observe anything directly.

I am not going to teach a lower level stats class in a comment section, but our physicist is correct in stating that this is a belief , because that is what it is - a extremely well tested belief, that we can consider to be "fact" in common parlance. You could of course chose not to believe it, if you can disregard all of human achievement.