opinion
[uh-pin-yuh n]
noun
1.
a belief or judgment that rests on grounds insufficient to produce complete certainty.
To most people, both supporters and opponents, evolution and global warming are a matter of opinion because they don't know enough for certainty. I suppose a lot of them could argue that it's actually a
fact
[fakt]
noun
4.
something said to be true or supposed to have happened:
The facts given by the witness are highly questionable.
While evolution and global warming are both definitely questions of fact rather than of preference, there are very few who could make that determination themselves rather than trust someone else's judgement.