The Fermi Paradox is the contradiction between the high possibility of life existing outside our planet and the lack of contact with those forms of life. Wait But Why has a truly great (but long) post explaining the Fermi Paradox. It's a great read and something to think about the next time you stare up at the sky.
My favorite part:
Possibility 9) Higher civilizations are here, all around us. But we’re too primitive to perceive them. Michio Kaku sums it up like this:
Lets say we have an ant hill in the middle of the forest. And right next to the ant hill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?
So it’s not that we can’t pick up the signals from Planet X using our technology, it’s that we can’t even comprehend what the beings from Planet X are or what they’re trying to do. It’s so beyond us that even if they really wanted to enlighten us, it would be like trying to teach ants about the internet.