Our galaxy, The Milky Way, has been around for nearly 14 billion years. It is home to at least 100 billion planets. About a third of these planets could potentially harbor life.

Scientists also hypothesize that an advanced civilization could colonize our galaxy in only a couple of million years. If this is true, the Milky Way should have been fully colonized many times. So why haven’t we seen aliens or even evidence of them? We call this the Fermi Paradox.

One theory to this paradox is The Great Filter. The Great Filter is a metaphorical roadblock in the hypothetical timeline of any advancing life form. This roadblock is practically impossible to get past. So, maybe all of the potential alien civilizations out there have encountered The Great Filter. 

This begs the question: Is the filter behind us, or in front of us? 

If the filter is behind us, then we are one of a small, recent selection—if not the first—to pass it. This would mean that some step in our evolutionary process is almost impossible to cross. This “impossible” step could be biological advances such as the creation of special cells to form such a complex brain. Either way, this scenario means we are the first to achieve this step. 

Of course, the second possibility is that the filter is in front of us and we are headed towards impending doom. There might have been civilizations before us that go even more advanced than humans. One of the speculated ideas for our roadblock is our technology turning against us. A second theory is that our barrier is intergalactic space travel, which is close to—if not fully—impossible. 

So the question is not whether we are the only life that exists. It is whether we are the only advanced civilization that exists. 

Note: When dealing with large numbers, it’s hard to fully understand the scale of which we are talking about. For reference—if every person on Earth was a planet in the Milky Way, the world population would need to increase by at least twelvefold.