To quote the wise Calvin:
"Sometimes I think the most sure sign that intelligent life exists elsewhere in the universe is the fact that it hasn't tried to contact us yet."
Ah, but the problem is that given any reasonable odds of life developing, we would have heard radio noise by now. Even if we a million years behind the other civilizations, their transmissions from their equivalent of the 20th and 21st Centuries would still be echoing through the Universe. So where are they?