I just read 'The Dark Forest, part 2 of a great contemporary sci fi series. It gives a very interesting take on the Fermi paradox. Spoiler for the book below. [spoiler]All species are forced into hostility by fear of each other; all strikes are pre-emptive. It is safer to maintain silence than to allow other species to know of your existence as they'll assume you could be a threat. So everyone hides.[/spoiler]