We're effectively alone in the Universe, and that's OK
Silence. Complete, unnerving silence. Despite decades of searches for any form of life, intelligent or otherwise, out there in the cosmos, the Universe has but one message for us: No one is answering.
But that solitude is not a curse. The great expanse of the empty heavens above us does not carry with it an impossible burden of loneliness. It begets a freedom--a freedom to explore, to be curious, to wonder, to expand.
The Universe is ours for the taking.
The great silence
According to physics legend, in the 1950s, the great scientist Enrico Fermi put it bluntly during a casual conversation with a friend: "Where is everybody?"
The logic behind the question is simple. Modern cosmology is built on the Copernican principle, or what I call the "Principle of We're Not Special." The Milky Way is an average, run-of-the-mill galaxy, one of hundreds of billions, if not trillions, in the observable volume of the cosmos. Our Sun is about as average as you can get for a star: middle-aged and middle-sized.
The Earth? OK, it's somewhat special. There's liquid water on the surface and a nice--but not too chokingly thick--atmosphere. Other worlds in the Solar System boast liquid water, too--it's just underground. And water is the most abundant chemical compound in the entire Universe, so we shouldn't be that surprised that it gets to be liquid here and there.
But even given that the Earth is pretty good, we're still not special. There's nothing that's obviously, triumphantly remarkable about the Earth, the appearance of life on it, or the eventual evolution of intelligent life. It happened here; it can happen anywhere. And given that the Universe is creeping on 14 billion years of age, life is bound to have arisen elsewhere.
But all those billions of years is more than enough time for some civilization to become extremely technically competent and send either themselves or their robotic emissaries throughout the galaxy, exploring if not outright colonizing every planet they wish. It's not like the Milky Way is that big. It's just 100,000 light-years across, so billions of years is plenty of time for someone to explore every little nook and cranny, even if they have to do it the slow way. Given these assumptions, evidence for alien civilizations should be obvious and manifest.
So we have a paradox: Where is everybody?
One answer is that we haven't looked hard enough. Obviously, intelligent life isn't super-duper common, considering that we're the only intelligent critters to arise in our own Solar System, and not every planet around every star will have the right conditions for life. So if intelligent civilizations aren't going to come calling, maybe we need to actively hunt for them.
In response to Fermi's paradox and at the urging of several prominent scientists like radio astronomy pioneer Frank Drake, SETI was born: the Search for Extraterrestrial Intelligence. The thinking behind SETI is that while intelligent life may be relatively rare in the cosmos, it would be exceptionally loud. Consider our own species as an example. As soon as we figured out the basics of electromagnetism and hit upon the concept of using radio waves to transmit information, we started blasting, generating radio emissions powerful enough to encircle the globe. And those radio emissions were truly omnidirectional, meaning that for every Earth-to-Earth transmission we generate, some of those radio waves make their way out into the vastness of space.