Which question has man found most difficult to answer?

Man has no trouble at all answering any question. If you point out that the answer is untrue, dishonest, or just stupid, he will shout insults at you until you give up and go away. No trouble at all.

Example: A man named Oort was studying bodies beyond the orbit of Pluto. He needed more than he could find, so he assumed them. Astronomers now assume that the Oort Cloud is a confirmed observation. That worked so well that he did it again: he needed mass to balance his equations, so he invented dark matter, and that is now considered a basic property of the universe even though by definition it can never be observed. Astronomers just assume it wherever they need it to balance their equations, and they claim the equations prove the existence of dark matter.