[Ichneumon]

["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.

Highly unlikely.

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.

No they wouldn't, radio/TV transmissions don't "echo through the Universe". Even if they did, after the first few years they would be so attenuated (remember our old friend, the Inverse-Square Law?) that there's no way we would be able to detect them with current technology.

So where are they?

For most plausible values of the Drake Equation, they're too far away for us to "hear" or for them to come visit us.

[Mr. Silverback]

We've already sent messages to likely stars. In a universe that almost certainly contains millions of planets, no one has decided to aim one at us, or aim one at another system that we end up overhearing?