Given the burgeoning evidence that there must be billions of Earth-like planets in our galaxy - let alone the observable universe! - why the heck have we never picked up any signals from other intelligent life? Enter the Fermi paradox.
So, are we utterly alone in the universe (or perhaps quite the opposite)? We don't yet have an answer to the question, and considering the matter inevitably involves some speculation and forays into the realm of science fiction. Nevertheless there are many possible resolutions to the Fermi paradox (as discussed in the linked article), and contemplating them turns out to be a deeply humbling and mind-bending experience.
Beyond its shocking science fiction component, The Fermi Paradox also leaves me with a deep humbling. Not just the normal "Oh yeah, I'm microscopic and my existence lasts for three seconds" humbling that thinking about the universe always triggers. The Fermi Paradox brings out a sharper, more personal humbling, one that can only happen after spending hours of research hearing your species' most renowned scientists present insane theories, change their minds again and again, and wildly contradict each other—reminding us that future generations will look at us in the same way we see the ancient people who were sure that the stars were the underside of the dome of heaven, and they'll think "Wow they really had no idea what was going on."