There are many aspects of the ancient world that we don't understand, and we marvel at some of the artifacts they have left behind. Whether it be the Nasca Lines in Peru, or the monuments left on Easter Island, there are a number of people who believe that aliens are behind some of these creations. Some argue that the ancient peoples couldn't have built such sophisticated structures with the technology that they had available to them. Did alien technology help with these structures, or have aliens ever influenced life on Earth?
This is where the Fermi paradox comes into play. Enrico Fermi once asked why there was no evidence of any aliens ever coming to Earth, despite the fact that the Earth should be a typical planet. Also, given the size of the universe, there had to have been some civilization come along and visit. Even if they couldn't visit, why haven't we even detected any other civilizations yet?
Why have there been no probes, no fossil evidence, no wreckage, or anything else in the fossil records of Earth? Is it because there are no civilizations around us, or are we just wrong about our assumptions of intelligent civilizations? Fermi argues that it should take about 5 million to 50 million years to colonize the galaxy. Seeing as how the universe has been around for about 13.7 billion years, someone should have colonized the galaxy by now.
This assumes of course that intelligent species are going to want to explore outside of the home planet. If most species are anything like humans, they would want to inhabit the areas around them. Even if there are no civilizations inhabiting the galaxy now, why don't we have any evidence of any civilization ever inhabiting the galaxy? Why are there no radio waves, or any other transmissions that would be left over from a civilization trying to make contact.
Do intelligent life forms want to explore the galaxy? Can they travel the galaxy? Based on the time scales involved, and the amount of planets that should hold life, why has Earth not been visited? Are we alone, or do civilizations just not have the ability or desire to come visit the Earth? Why, if there are aliens nearby, would they not want to visit Earth out of curiosity? Can they not get here, or do they just not want to necessarily discover every planet they can?
Essentially the Fermi paradox argues that if aliens exist, why don't we see them? It should be possible for aliens to populate the galaxy by now. Even if they don't exist anymore, there should be evidence that they did exist that we can detect. Even if life is rare, the age and size of the universe says there should be something out there. So where are they, and why can't we see them? Can they just not get here, or do they not exist? The Fermi paradox continues.