Fermi Paradox
The Fermi paradox highlights the contradiction between the absence of definitive evidence for advanced extraterrestrial life and the high probability that such life exists. Proponents argue that given the favorable conditions for life on Earth, extraterrestrial life should be common, making it unlikely that we haven't detected it.
#6 Actually a really cool list
#19 I remember when I first found out that the Roman statues used to be paint to make them appear lifelike. Creepy.