Maybe life is out there, and we simply missed it.
Consider two factors: size and time.
SIZE
Is there a chance that the "life out there" is so small (compared to our own) among the vastness of the universe, that we are unlikely to see it even if we are looking right at it?
Brings to mind a cartoon (always a good reference
) on the subject that I once saw. Some guy is sitting at a table. On the floor beside him is a tiny space capsule the size of a pea, with a tiny door open on it. He's looking through a microscope, and a bunch of cells in the viewfinder are lining up to spell out the words, "Take us to your leader".
TIME
Some people say the Earth is maybe 4.5 billion years old, and the rest of the universe is much older. How long has Man existed on the planet? For that matter, how long has ANY "life as we know it" existed here, from single cells thru trilobites thru T-rexes thru man? Just a tiny fraction of the life of the planet. And if we start tossing nukes around or some other thing, that could all be gone in a thousand years or less.
"Life as we know it" has only existed HERE for a blink of an eye, in the history of the universe. Who's to say some other life form didn't develop or get created (insert your own favorite theory of origin here), half a universe-lifespan ago, grow, invent machines, spread through a galaxy or two, then goof and wipe themselves out like we might, all in their own relative blink of an eye, say ten billion years ago? Would there be ANY evidence of that left now? Even if there were, what are the chances of our finding it? We've been listening (and broadcasting) for less than a hundred years. And we've sent out what, a grand total of three spacecraft to exit our own SOLAR SYSTEM, and only in the last 20-30 years?
We have ZERO evidence that we are the only ones who are here, or have ever been. Course, there's zero evidence that we aren't, too. There's a very good chance that, if there was ever "life" in the universe, we (inevitably) missed it, and it's long gone now. If it ever existed.
What chances are there that, many billions of years in the future, other beings someplace will be saying the same thing... never knowing they are referring to US?