Why Haven't We Seen Aliens (PB)

If there really is alien life elsewhere in the Universe, then it''s reasonable to ask why we haven''t seen it. Surely planet Earth must have been visited by aliens, if they exist. Does that mean that alien life doesn''t exist? This book uses our knowledge of the universe, some simple maths and a little bit of intelligent speculation to take a sober look at the chances of our actually meeting alien...