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

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?

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.

This is the most optimistic interpretation. Maybe we're so clever and rare that we somehow found our way past the Great Filter while all other lifeforms have failed.

There's no way to tell what critical moment in our history may have gotten us past the filter though. It may have happened at very beginning of life. Maybe the great filter is the likelihood that life arises at all in the first place.

Or it could be the jump from simple cells to complex ones. Or the jump from semi-intelligent animals (apes) to intelligent hominids.

If you don't buy into the idea that we're so rare and special that we're the only ones to make it past the great filter, there's another pretty optimistic view point to which you can subscribe.

Maybe it's pretty easy to make it past the great filter, but the conditions in the universe have only just become ripe to support intelligent life that makes it to the other side.

That would make us the first form of life that has a chance to survive beyond the great filter. We're simply in the right place at the right time to have a shot.

Being in the right place at the right time would take a lot of luck. So if you don't think we're past the great filter, and you don't think we're the first life forms with a shot at making it past the great filter, there's only one remaining conclusion: Humans are toast.

A cataclysmic natural disaster, such as a huge asteroid impact or gamma ray burst, could obliterate the planet. Or we could invent some kind of super-advanced tech that will destroy us all.

That's why some scientists and philosophers are praying that we don't discover even simple microbial life somewhere like Mars or Europa. If we find life elsewhere, that will mean that it's much more likely that the great filter is still ahead of us and we're headed for disaster.

"So this is why I conclude that the silence in the night sky is golden, and why, in the search for extraterrestrial life, no news is good news," Oxford philosophy professor Nick Bostrom writes on his website. "It promises a potentially great future for humanity."

Let's now move past that depressing "great filter" idea of inevitable doom.

Instead, we can consider the possibility that extraterrestrial life is abundant — but for a number of reasons, we haven't been able to get in touch with any of it.

Humans have only existed for about 200,000 years out of the Earth's 4.5-billion-year existence. That's just a tiny sliver of our planet's timeline.

Aliens might have visited the Earth when is was still a molten ball of lava or a stew of primordial soup.

Or, for all we know, the dinosaurs might have seen some crazy aliens that left long before humans arrived on the scene.

When Europeans landed in the Americas, it took a long time before they reached some distant Native Americans living far out on the West Coast.

It might be possible that alien civilizations just haven't traveled far enough to stumble upon us yet. Sort of like the remote ice planet Hoth in "Star Wars," or the wild west of space travel, as is portrayed in the TV show "Firefly."

Intelligent alien life may have discovered a way to upload their minds into a completely self-sustaining virtual reality world where they don't need access to outside resources or energy.

They're perfectly content in the world they created, and not only completely unconcerned about getting in touch with us but reticent to do so.

It's possible we haven't heard from anyone else because they know better to broadcast any signals, or at least encrypt them (as Snowden suggests) — lest one of these killer civilizations pick it up. That's why some experts are vehemently opposed to a Messaging to Extraterrestrial Intelligence (METI) mission: the controversial next step to SETI, where instead of just passively listening, we actively send messages out into the cosmos.

You can take this creepy idea even farther. Maybe the first civilization that managed to achieve super intelligence is the ruler of the universe and it's out there destroying anyone else that gets close to its level.

So unfortunately, once again, we might be on a dangerous path — this time by blasting any and all communications into space.

Aliens might be actively broadcasting signals and trying to communicate with us, we just might not have the means to detect any of it.

Maybe we can't access the right frequencies. Maybe we don't have the right technologies yet. Maybe other life forms only communicate via telepathy.

Or perhaps super-advanced aliens might exist, and they don't want to blow our minds by visiting us with their incomprehensible technology and intelligence. They're simply waiting until we catch up, treating us as an exhibit at a zoo — look but don't touch...

If you've seen Interstellar, think about the aliens that exist in a fifth dimension and built a wormhole.

Like a person trying to communicate with a bug, we may not be able to comprehend what aliens are at all. In this scenario we're completely irrelevant to them.

We could be living in the Matrix, and we were put here by another super-intelligent race as an experiment. Or maybe we're all just one big computer simulation.

The bottom line is we don't know. The most intelligent people in the world have wildly different opinions about the answer to the Fermi paradox.

Humanity could be completely alone — the king of the cosmos, as our Earth-bubble has already led us to believe. Or other Type II and Type III lifeforms exist and we're a tiny, insignificant piece of a vast universe full of life. It's humbling, but also oddly comforting to consider that there could be far more to existence than we think there is.

Whatever the answer is, there's a lot to think about the next time you look up at the night sky.