We’ve all seen the doomsday sci-fi movies.

Aliens come down and threaten to destroy our world, and it’s up to the collective might of humanity to defeat the interstellar invaders.

Yet, thankfully, these scenarios have yet to be realized.

According to Space.com, there are between 100 to 400 billion stars in the Milky Way galaxy alone. In addition to this vast amount of stars, there are approximately 100 billion planets. Of those stars that are like our sun, about 22 percent have planets about the size of Earth and are potentially habitable.

These numbers don’t even come close to capturing the size of the Observable Universe, which contains hundreds of trillions of stars and planets. Yet with all this data showing that within the immense reaches of space, there should be someone else out there, our little blue world hasn’t received a message from the void.

So where are all the aliens?

This is the basis of the Fermi Paradox. It is the problem of why we haven’t made contact with aliens, despite the statistics being on our side. This has sparked debate amongst sociologists, astrobiologists, ethicists, sci-fi writers and everyone in between. The result of this debate has not provided a conclusive answer, but it has provided possible solutions.

The Great Filter

Throughout the history of life on Earth, there have been five mass extinctions. Each one ended from 75 to 96 percent of all life.

Despite these periods of extreme devastation, life pulled through. This has led people seeking to answer the Fermi Paradox to conclude that there is something known as the Great Filter. This filter represents a barrier in the process of the development of complex life that is very difficult to pass.

From this theory, there are two possibilities. The first is that life on Earth has already passed the Great Filter, and we will go on to progress and thrive. If the Great Filter does exist, this is the best outcome. To prove that we have passed the Great Filter, we would need to find fossilized primitive life on other planets. This would show that the Great Filter is something that happens early on in the development of life and that we have made it.

The second possibility is much more grim. In this scenario, we have yet to reach the Great Filter. This filter occurs during the development of complex civilizations. To prove we have yet to pass the Great Filter, we would need to find the ruins of ancient super-advanced civilizations. If this is the case, humanity is most likely doomed to the same fate.

We Just Don’t Notice Them

According to the Kardashev scale, there are three types of civilizations. A Type-I civilization is a civilization that has access to the total energy of its planet — according to astrophysicist Carl Sagan, we are at about .73.

A Type-II civilization has access to the total energy of its home star. This would involve the construction of something known as a Dyson Sphere, which is a device that is placed around the star and powers the civilization through its energy output. A Type-III civilization is able to access the total energy of its galaxy.

A Type-II civilization, needless to say, would be way beyond our understanding. A Type-III civilization, on the other hand, would be god-like. Perhaps a Type-III civilization already exists and simply pays our little solar system no mind. On the other hand, does an ant on the ground in New York know about the Empire State building? Perhaps a Type-III civilization is right on our doorstep, and we just simply can’t recognize them.

All in all this solution could play out either way for humanity. Perhaps they don’t mess with us like an ant hill in a field. Or they just simply destroy us ants in the process of building their post-office, paying no mind to our concerns or wishes. Either way, it’s a quite scary thought.

The Dark Forest

This scenario is quite unsettling. In this solution, life is actually quite abundant in the universe, but nobody wants to talk to each other.

This is because they simply don’t know how other civilizations will respond to their existence.

Like hunters in a forest, they have no knowledge of each other’s motivations or goals. And even if they stated them outright, how could you trust them?

In a universe of finite resources with an ever-expanding population, survival comes first. So when one civilization discovers another, rather than risk their own destruction, they destroy the other civilization unlucky enough to have revealed itself.

Unfortunately for us, our radio signals have now covered a distance of around 100 light years. Within this circle of 100 light years in diameter, there are many star systems, including the Alpha Centauri star system, which is only four light years away.

Perhaps a civilization has heard our signals and is preparing to destroy us. Or perhaps they’re less advanced and are not responding out of fear of us destroying them.

In a twist on this scenario, instead of a forest of hunters, there is only one hunter and the rest are prey. There may be a significantly advanced civilization that goes around destroying life once it is advanced enough to send out signals into space, and the reason we aren’t hearing anyone is because they are hiding.

Either way, let’s hope the universe is not a dark forest.