[Excerpt from a book on the existence of extraterrestrial intelligence, by Dr. Robert James III]
Consider that there are billions of stars in the galaxy like our sun, many of them far older. Some of these stars will have Earth-like planets. Some of these planets may have developed intelligent life. Some intelligent life may develop interstellar travel.
If a civilization began before ours, it seems possible - even probable - that even at the speed of currently envisioned interstellar travel, they could already have the entire galaxy colonized.
The problem is, we haven't detected any such civilizations. Nor have any contacted us.