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Ever since this video came out in May, a puzzle that has bugged scientists since we started sending out radio signals has regained popularity online: when you think about how big the universe is, it's really weird that we haven't detected any aliens.

Aliens. On the one hand, anyone who says they've seen or met them is probably a liar or an idiot. On the other hand, mathematically speaking, the universe should be crawling with them. The universe is both big and old, meaning that life should have had plenty of time and chances to develop, advance, and spread across each galaxy. The fact that it hasn't, as far as we can see, either means we're alone (unlikely), most life goes extinct before then (depressingly likely), or there's a horrifying race of super-robots out there that will wipe us out once we're advanced enough (unlikely, but would make a pretty good video game series). This problem, also known as the Fermi Paradox, is the subject of the video above by animator and explainer Kurz Gesagt, and since debuting on May 6, it's sparked a small renaissance on the topic.

Sources: Kurz Gesagt - In a Nutshell