Where is everybody? They're dead.

Fermi's Paradox was first posed by physicist Enrico Fermi in 1950. It goes like this: The universe is many billions of years old. Fermi calculated that an alien species smart enough to become spacefarers could reach any point in our galaxy in five million years. But we we have no scientific evidence that aliens beings have been here.

So, Fermi asked, where is everybody?

Many answers have been proposed by serious, highly-credentialed scientists - more than 50 different answers, as I recall. Now, Astronomy.com offers this: The aliens are silent because they are extinct:

Latest theory: This will never hear anything.

Life on other planets would likely be brief and become extinct very quickly, said astrobiologists from the Australian National University (ANU).

In research aiming to understand how life might develop, scientists realized new life would commonly die out due to runaway heating or cooling on their fledgling planets.

“The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens,” said Aditya Chopra from ANU.

“Early life is fragile, so we believe it rarely evolves quickly enough to survive.”

“Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable.”

About four billion years ago, Earth, Venus, and Mars may have all been habitable. However, a billion years or so after formation, Venus turned into a hothouse and Mars froze into an icebox.

Early microbial life on Venus and Mars, if there was any, failed to stabilize the rapidly changing environment, said Charley Lineweaver from ANU.

“Life on Earth probably played a leading role in stabilizing the planet’s climate,” he said.

Then there is recent study, published in the prestigious journal Science, that life is simply impossible in probably 90 percent of galaxies in the universe because of intense gamma radiation. And ordinary solar and cosmic radiation would have stopped life here on Earth without the Earth's magnetic fields shielding the planet, but planetary magnetic fields apparently are very uncommon; they have not been detected on any other planet anywhere.

But wait! There's more! The Atlantic writes of World War 2 bomber crews who did pattern analysis of bullet holes from enemy fighters attacks. They thought if there was a pattern, then they could lessen the number of bombers shot down by increasing the armor in the hit sections of the bomber.

But it turns out that armoring holed sections of returned bombers was the wrong thing to do. 

Don’t protect the planes where they were taking the most damage, [mathematician Abraham] Wald said. Armor the planes where there were no bullet holes at all.

“You put armor where there are no holes, because the planes that got shot there didn’t return to the home base,” says Anders Sandberg, a senior research fellow at University of Oxford’s Future of Humanity Institute. “They crashed.”

The article goes on to explain "observer selection effect," where we are able to observe something only because we survived the causes. We look at our own world and see life in enormous variety, flourishing everywhere, even in rocks and immense pressures of the deep sea and hot springs of near-boiling temperatures.

So we readily conclude, "Life is everywhere on our planet, so it must be everywhere out there." This powers the SETI programs, in fact, and is so pervasive it even has a name: the Principle of Mediocrity, which means simply that earth and its biosphere are unexceptional. The earth and its life are merely average in the universe - average, which is what "mediocre" means. But it is just as likely - probably more so - that our conclusions spring the the observer selection effect: we conclude that what we see here is normal everywhere.

We see 100-mile-wide "bullet" holes on our planet, and hey, we're still here. All is well and this is cosmically normal. But there's a problem.

After all, there are 100-mile impact craters on our planet’s surface from the past billion years, but no 600-mile craters. But of course, there couldn’t be scars this big. On worlds where such craters exist, there is no one around afterward to ponder them. In a strange way, truly gigantic craters don’t appear on the planet’s surface because we’re here to look for them. Just as the wounds of the returning planes could reflect only the merely survivable, so too for our entire planet’s history. It could be that we’ve been shielded from these existential threats by our very existence. ...

 “Maybe the universe is super dangerous and Earth-like planets are destroyed at a very high rate,” Sandberg says. “But if the universe is big enough, then when observers do show up on some very, very rare planets, they’ll look at the record of meteor impacts and disasters and say, ‘The universe looks pretty safe!’ But the problem is, of course, that their existence depends on them being very, very lucky. They’re actually living in an unsafe universe and next Tuesday they might get a very nasty surprise.”

If this is true, it might explain why our radio telescopes have reported only a stark silence from our cosmic neighborhood. 

"Stark silence." Where is everybody? They're dead.

Perhaps we’re truly extreme oddballs, held aloft by a near-impossible history—one free from deadly migrating gas giants and solar-system chaos, but also filled with freakishly favorable accidents, like a cataclysmic impact early in our history that created a strange, gigantic moon that stabilized our orbit and allowed complex life to flourish. As the solar system continued to shake out, we somehow ended up with just the right amount of water to lubricate plate tectonics, keeping the climate habitable over hundreds of millions of years and preventing a Venus-style planetary resurfacing catastrophe, but not so much water that we wound up on a lifeless water world.

So far, empirical evidence supports the conclusion that we are alone.