Basically discussing the Fermi Paradox (http://en.wikipedia.org/wiki/Fermi_paradox) and the Great Filter (http://en.wikipedia.org/wiki/The_Great_Filter) with further points below.
Nick Bostrom is the director of Future of Humanity Institute at Oxford University, which is working on some fascinating questions that almost cross into the line of sci-fi. (Check out http://www.fhi.ox.ac.uk/ ) They published a book called "Global Catastrophic Risks" which analyzes "Global catastrophic risks are risks that seriously threaten human well-being on a global scale. An immensely diverse collection of events could constitute global catastrophes: potential factors range from volcanic eruptions to pandemic infections, nuclear accidents to worldwide tyrannies, out-of-control scientific experiments to climatic changes, and cosmic hazards to economic collapse."
Nick Bostrom writes in an essay entitled "Where Are They? Why I hope the search for extraterrestial life finds nothing" why he hopes we find nothing because of what he calls a "Great Filter". The Great Filter that prevents other civilizations entering space might be behind us, or ahead of us he states. Such as the improbability of evolution of intelligent life in our history. The Great Filter ahead of us gets more interesting.
He says "The other possibility is that the Great Filter is after us, in our future. This would mean that there is some great improbability that prevents almost all technological civilizations at our current human stage of development from progressing to the point where they engage in large‐scale space‐colonization and make their presence known to other technological civilizations. For example, it might be that any sufficiently technologically advanced civilization discovers some technology—perhaps some very powerful weapons technology—that causes its extinction."
He feels that if there were space traveling civilizations anywhere near us we would have already found it, or it us. Through either a probe, radio waves, or some observable event. He does not rule out the possibility that advanced civilizations have chosen to hide themselves from us, but considers it unlikely. He explains why he feels if space faring civilizations exist we would have encountered them or be able to see them.
In relation to why we would see space faring civilizations if existent, "First, life here on Earth manifests a very strong tendency to spread wherever it can. On our planet, life has spread to every nook and cranny that can sustain it: East, West, North, and South; land, water, and air; desert, tropic, and arctic ice; underground rocks, hydrothermal vents, and radioactive waste dumps; there are even living beings inside the bodies of other living beings. This empirical finding is of course entirely consonant with what one would expect on the basis of elementary evolutionary theory. Second, if we consider our own species in particular, we also find that it has spread to every part of the planet, and we even have even established a presence in space, at vast expense, with the international space station. Third, there is an obvious reason for an advanced civilization that has the technology to go into space relatively cheaply to do so: namely, that’s where most of the resources are. Land, minerals, energy, negentropy, matter: all abundant out there yet limited on any one home planet. These resources could be used to support a growing population and to construct giant temples or supercomputers or whatever structures a civilization values. Fourth, even if some advanced civilization were non‐expansionary to begin with, it might change its mind after a hundred years or fifty thousand years—a delay too short to matter. Fifth, even if some advanced civilization chose to remain non‐expansionist forever, it would still not make any difference if there were at least one other civilization out there that at some point opted to launch a colonization process: that expansionary civilization would then be the one whose probes, colonies, or descendants would fill the galaxy. It takes but one match to start a fire; only one expansionist civilization to launch the colonization of the universe."
