The likeliest reasons why we haven’t contacted aliens are deeply unsettling

Even on the clearest, darkest night far from city lights, you can see only about 1% of the Milky Way galaxy's 100 billion to 400 billion stars.

Here's the real trip though: For every star in the Milky Way, there's a unique galaxy drifting through the universe, each with it's own billions of stars, and approximately one planet orbiting each of those stars. That's billions and billions and billions of worlds.

And yet decades' worth of missions by Search for Extraterrestrial Intelligence (SETI), an organization which listens for signs of life in space, have come up completely empty handed. Every. Single. One.

Physicist Enrico Fermi is famous for posing the natural question that follows: Where is everybody? The scale of the universe and basic math tell us alien life must be common, yet there's no evidence for it.

Welcome to the Fermi paradox.

Philosophers, physicists, and astronomers have tried to answer the Fermi paradox since its unofficial inception in 1950. Even Edward Snowden, a digital surveillance expert and former NSA contractor, recently shared his best explanation on StarTalk, astrophysicist Neil deGrasse Tyson's podcast.

These and other answers proposed by experts are deeply unsettling — especially if you spend too much time researching them, like I did.

Keep scrolling down to get a little background on why it's so inconceivable we are utterly alone in the universe, and why it's so spooky we have yet to hear from anyone.

Aliens 101: The Kardashev Scale & Fermi Paradox

Think about how far humanity has progressed in its short 200,000 years of existence. Now consider that our galaxy is roughly 10 billionyears old.

If we can go from cave-dwelling hominids to an internet-using and robot-building society in 200,000 years, what could an alien race achieve in 10 billion years?

That's more than enough time for a civilization to develop sophisticated rockets — possibly faster-than-light travel, wormhole technology, or some other kind of cosmic shortcut that would allow them to rapidly colonize the galaxy and beyond.

The Kardashev Scale, created by astrophysicist Nikolai Kardashev, is helpful when considering such technological advancement by a developing civilization. It has three types:

1. A Type-I civilization has figured out how to harness all the energy on its planet. Humans are getting close to achieving this, but that's just the first tier.
2. Type-II civilizations are so intelligent they've figured out how to harness all the energy of their own star — an incomprehensibly larger amount of energy than what's available on one puny planet.
3. That's nothing compared to the Type-III civilizations, though. Those have harnessed all the energy available in their galaxy.

Any type of civilization on the Kardashev Scale would be more than capable of colonizing the universe. But we haven't detected any of these civilizations — and that's the heart of the Fermi Paradox.

You can group the best explanations for the paradox into two distinct categories: one in which aliens don't exist, and we're completely alone in the universe, and one in which aliens do exist, but for some reason we haven't made contact.

Let's start with leading ideas in the former category.

Aliens don't exist #1: The Rare Earth Hypothesis

Life on Earth might simply be a freak accident of nature, and it may not exist anywhere else in the entire universe.

This idea is called the rare Earth hypothesis. It suggests a perfect storm of things like Earth's protective magnetosphere, temperature, size, axis tilt, etc., all came together to create a very precise cradle for life to arise. These are the only conditions that life can exist in, and they don't exist anywhere else.

While it's certainly possible, the odds aren't in this idea's favor — the universe is far too vast.

For example, the European Space Agency estimates there's about 1,000,000,000,000,000,000,000,000 (or 10²⁴) stars in the observable universe.

There's no scientific consensus as to how many of those stars might be like our own Sun, and how many may have Earth-like planets orbiting around them. But if you take even the most conservative estimate out there, then about 5% of those 10²⁴ stars are like the Sun. That means there are 500 billion billion other Sun-like stars.

Next, if you take the lowest estimate of how many of those Sun-like stars have an Earth-like planet orbiting it (22%), that means about 100 billion billion other Earth-like planets are out there.

Put another way, roughly 100 Earth-like planets exist for every grain of sand on Earth. Surely one of those would have life on it?

Aliens don't exist #2: The Great Filter

If Earth isn't the only planet capable of supporting life, and there are definitely no aliens out there, then something grim is going on.

That something is called the great filter, and it's kind of terrifying.

The idea is that before a civilization can make it very far along the Kardashev scale of intelligence, it hits a wall — a filter — and it dies. That's why we haven't heard from anyone: Life regularly evolves to where we're at here on Earth, then some powerful, inevitable force snuffs it out. (Nuclear weapons? Overpopulation? Asteroids? Disease?)

The problem is that there's no way to know where on the timeline of life that the great filter sits. Did we already make it past the filter, or are we are on our way to inevitable doom?

There are three possible scenarios to explain why we're still here.

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