More and more astronomers are coming around to the idea that we’re not alone in the universe. To them, it’s a matter of math, and humility. With potentially trillions of life-supporting planets out there, why would ours be the only one to evolve a high-tech civilization?

But if extraterrestrials do exist, we still haven’t met them yet. (Probably.) You’d think out of trillions of chances for life to spawn in the universe, we’d have found signs of other intelligent life by now, right?

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Now a team based at NASA’s Jet Propulsion Laboratory in California is revisiting an old theory to explain why. The “Great Filter” theory posits that other civilizations, potentially many, have existed during the history of the universe, but they all wiped themselves out before they got a chance to make contact with us.

Read more: Daily Beast