Fermi Paradox

"The Fermi paradox…is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations. The basic points of the argument, made by physicists Enrico Fermi and Michael H. Hart, are:

•  The Sun is a typical star, and relatively young. There are billions of stars in the galaxy that are billions of years older.
•  Almost surely, some of these stars will have Earth-like planets. Assuming the Earth is typical, some of these planets may develop intelligent life.
•  Some of these civilizations may develop interstellar travel, a technology Earth is investigating even now (such as the 100 Year Starship).
•  Even at the slow pace of currently envisioned interstellar travel, the galaxy can be completely colonized in a few tens of millions of years.

According to this line of thinking, the Earth should already have been colonized, or at least visited. But no convincing evidence of this exists. Furthermore, no confirmed signs of intelligence…elsewhere have yet been spotted in our galaxy or (to the extent it would be detectable) elsewhere in the observable universe. Hence Fermi's question, "Where is everybody?""

--WIKIPEDIA, Fermi paradox