We Are Most Likely Alone in the Universe
"Where are they?" famously asked Italian physicist Enrico Fermi in the 1950s. By "they," he meant space aliens. Fermi figured that if the galaxy contained space-faring civilizations it would only take them a few tens of millions of years to populate it. So why hasn't there been a saucer-landing-take-me-to-your leader moment already? This is the Fermi Paradox.