The universe is really old and really big. It has existed for at least 13.7 billion years, and the part that we can observe has a diameter of 93 billion light years. It is filled with roughly 10 trillion trillion planets, of which 100 billion billon are Earth-like in temperature. There is no reason to think that it ends at the boundary of what we can observe, meaning that it might be infinitely large. The question follows – do any of those other planets have life?

Extraterrestrial life is an extremely common idea in science fiction. However, scientists have not found any direct evidence of it – not for lack of trying. Many have tried to puzzle out why we have not found anyone yet and why no one has contacted us. After intense head-scratching, they have spelled out at least 21 possible explanations for what has become known as the "Fermi paradox." They have tried to answer a simple question: "Where is everybody?"

Describing all of them in detail would take a while, so here is a quick overview of ten of the most interesting possibilities:

1. It is the nature of intelligent life to destroy itself.

Starting on an optimistic note, it is possible that alien civilizations have risen before and subsequently destroyed themselves. Some of the explanations for that include war, environmental destruction and an artificial intelligence uprising. Good thing none of those are a problem for us, right?

2. Extraterrestrial life is rare or non-existent.

This argument, also known as the "Rare Earth hypothesis," claims that life-friendly conditions are so absurdly improbable that they have only happened a few times – or maybe even once. Apparently, it is possible that our planet is the only one among one billion billion – or infinity, depending on how much more universe exists that we cannot observe – similar planets in the last dozen billion years or so which has ever had a civilization develop.

3. Intelligent civilizations are too far apart in space or time.

Consider that it is impossible to anything to travel faster than light, barring hypothetical ideas like wormhole travel and warp drives. Taking the universe's incredible bigness into account, if alien civilizations exist, it would make sense for them to be too far apart to communicate.

4. Earth is deliberately not contacted.

This concept, also known as the "Zoo Hypothesis," basically states that we are an exhibit in a cosmic zoo. There are alien civilizations, but they do not contact us for some reason. Maybe they wait for other civilizations to pass a certain technological or cultural standard before contacting, like Star Trek's "Prime Directive."

There is another reason we know of that they may hesitate to contact outsiders...

5. It is dangerous to communicate.

Maybe aliens do not want to risk contact with other aliens such as us. Our own history can shed light on this point. Generally, when two civilizations meet for the first time, one of them subjugates the other – and it would be insane to intentionally look for subjugation.

6. Civilizations tend to isolate themselves.

A species with advanced enough technology to explore the galaxy may have advanced enough technology to simulate a galaxy in virtual reality, and prefer the latter. Or they might just not care enough about the universe outside their own civilizations – who knows what alien psychology would be like?

Speaking of alien psychology...

7. They are too alien for us to comprehend.

Aliens may have a thought process that is so slow or so fast that we would not even recognize it. They might be so far beyond us that we could not even comprehend who or what they are, or any attempt that they made to communicate.

8. We are not listening correctly.

Consider how much communication technology has changed over the past century. Why should we assume that aliens would use something as old-fashioned as radio, in the specific frequencies that we can detect?

9. Everyone is listening, but no one is transmitting.

Humans have put far more effort into detecting alien life than making ourselves detectable. Who is to say that aliens do not do the same? Maybe there are plenty of aliens who would be happy to meet us, but they have not sent any messages. In our case, it is the official policy of the Search for Extraterrestrial Intelligence (SETI) organization not to reply to any alien message until "appropriate international consultations have taken place," which could be a bureaucratic nightmare.

Let's finish with a favorite possibility amongst conspiracy kooks.

10. Aliens are here unacknowledged.

Maybe aliens have contacted us, but the government is keeping it secret. That seems to make sense, considering how monumental of a discovery that would be. However, scientists usually say that the opposite happens – news articles are eager to claim that any new discovery is evidence of aliens, but the scientists themselves are far more cautious and skeptical.

Everything is either speculation or raw statistics at this point though. We may never be able to explain the paucity of evidence of alien civilizations, and the true explanation will likely be a combination of multiple different explanations. It is still fun to think about though.

For more information, check out some of the other Odyssey articles on this subject:

James Sayen:"Fermi Paradox Part One" (He also has several other great articles about astronomy.)

Joshua Elias: "Where Is Everybody: Aliens And The Fermi Paradox"

Arianna Broad: "The Fermi Paradox: Where Are They?"

Azure'Rea Hike: "Statistically, Aliens Do Exist"