Scientists have some ideas about why we haven’t found aliens yet. They think there are two possibilities: either there’s a lot of life in the Universe, or almost none at all. A study published in the International Journal of Astrobiology gives some reasons for this mystery.

Scientists searching for aliens believe that there should be many advanced civilizations out there besides ours. They used math and logic, based on the work of John Haldane, to explain why we haven’t found any extraterrestrial civilizations yet.

Imagine a group of planets like Earth. You’d expect life to appear on all of them or none of them. There’s no clear reason why only half of these planets would have life.

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Scientists believe that if alien civilizations ever existed in the Milky Way, they could have spread across the galaxy in 10-100 million years. Even if they disappeared, we should still find evidence of their existence. Since we haven’t found anything, it seems like alien civilizations are rare and don’t last long.

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The authors also suggest that if we ignore the lack of evidence, the Drake equation shows that the Universe might be full of life or almost empty.

If the Universe is full of life, F would be close to 1, and this could still be possible under certain conditions.

Maybe alien civilizations stay in their own part of space, and our solar system isn’t a place they visit. Maybe traveling between stars is too hard, or they just don’t need to do it.

It’s also possible that while there’s a lot of complex life in the Universe, there are very few technologically advanced civilizations, or it takes a long time for them to develop.

There’s even a chance that the birth and death rates of alien civilizations haven’t balanced out yet, so we might still have time to find them. But, considering how old the Universe is, this seems unlikely.