The Fermi Paradox is the seeming contradiction between the high estimated probability that extraterrestrial life exists, and the lack of evidence for any such life so far.
If you can’t see fish (analogous to the aliens), they’re probably camouflaged. Why would that be necessary? Perhaps a predator is doing the rounds? The most likely threat to civilisation on earth, apart from our good selves, would be another civilisation.
This is actually quite a terrifying comic.
See the original on the xkcd website. Want more explanation? Try here.
