Why We Can't Find Aliens

The Fermi paradox - there should be life elsewhere in the universe - but we can't find any.

Very intersting treatment of the question is in the following article, including connection to current developments such as nano and femtotech -

http://www.veronicasicoe.com/blog/2015/06/blowing-up-the-kardashev-scale-the-black-hole-alternative/