The Fermi Paradox is endlessly fascinating to me. The paradox is simple--there are somewhere between 100 billion and 400 billion stars in our galaxy, cosmologically speaking our Sun is relatively young, and our species is brand new. If life is abundant in the universe, why haven't we heard from anyone else yet?
This article does a good job breaking down the possible reasons we haven't made contact yet. The explanations range from the macabre to the comical. Personally, I'm optimistic that the first species that developed interstellar travel hasn't been wiping out any other species that can potentially compete with them. I also would hope that we don't just live "far out in the uncharted backwaters of the unfashionable end of the Western Spiral Arm of the galaxy." Getting any resolution to the Fermi Paradox would frankly be terrifying, but it's a good excuse to spend a few minutes this weekend laying with your back on the ground and contemplating the infinite.