In other words, we could never correctly say 'there is no evidence of unicorns on earth', because evidence of unicorns might be lurking behind the bushes on the south-west corner of Central Park, NY. In fact, the phrase "there is no evidence of..." could never be correct, according to darbymcgill.
Interestingly, I googled the literal phrase "There is no evidence of" and found 4,170,000 hits. All wrong, according to our hero. Among the nonentities who rashly used this nonsensical expression were NASA (several times), Columbia University, the UN, George Bush's physician (he said there was no evidence of heart disease; poor W probably thought that was a good thing) and W himself.
Thanks for setting the entire world straight, darby!