With billions of planets capable of supporting intelligent life, why no visits to earth? This is "The Fermi Paradox." Any civilization, with modest rocket tech & immodest funds, could rapidly colonize the Milky Way...
Extraterrestrial life should be common. Discussing this with colleagues over lunch in 1950, physicist Enrico Fermi asked: "Where are they?" or "Where is everybody?" The paradox is between evidence for its likelihood, and lack of evidence for its existence
E. Leonel, S. Kamphorst, S. Carvalho, and J. Silva. Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)