[andreftavares]

If you consider the "infinite" of space and, especially, time, I believe that the probability of the existence of (at least) one "inteligent" species capable of reaching any given place of the universe is 100%.

The question of why they don't contact us is simple: Prime Directive (http://en.wikipedia.org/wiki/Prime_Directive)

Disclaimer: I'm not a particular fan of Star Trek and bumped into this interesting principle, which I believe makes perfect sense.

[jerf]

Time is not infinite, and our past light cone is not infinite. It grows as less than the cube of the space we can observe (the universe is very not universally distributed), while the independent probabilities of whatever is required for life multiply exponentially. The idea that the odds of life are 100% is just a naive "Big Numbers" fallacy.

The Prime Directive was created to preserve the desired structure of drama on a TV show. It is ludicrous from any other perspective.

[NathanKP]

The Prime Directive only works if you assume that these hypothetical species are much more altruistic than we ourselves are. I find it highly unrealistic that any species, however highly evolved, would be 100% altruistic, with no rebels/thieves/slavers/prospectors that would desire to interfere with or destroy us humans.

And that's not to mention that realistically it would be almost impossible for a civilization to completely hide its presence. For example as humans our radio footprint from TV and radio and satellite broadcasts is spreading out of our solar system. That's going to keep on traveling and I don't see any way that we'd be able to just "erase" that so that no one else would notice it.