The Fermi Paradox is the apparent contradiction between the high probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations.
The paradox is named after physicist Enrico Fermi, who posed the question "Where is everybody?" in 1950 during a conversation about the possibility of extraterrestrial life. Fermi's argument was that if intelligent life exists elsewhere in the universe, then it should have had time to colonize the entire galaxy, or at least send signals that we should have detected by now.
There are several possible solutions to the Fermi Paradox. One solution is that intelligent civilizations tend to self-destruct. This theory, known as the "Great Filter," suggests that there may be some universal obstacle that prevents civilizations from advancing beyond a certain point.
Another solution is that intelligent civilizations are common and do communicate with each other, but we have not yet detected their signals. This suggests that we may not yet have the technology to detect such signals, or that the signals are being intentionally masked or hidden.
A third solution is that there are galactic civilizations, but they destroy any civilization which gets too powerful. But if there is a civilization that wants to eliminate other civilizations it is more likely that it will be motivated by culture, rather than by economics. In that case it will be more effective to automate the process by constructing a self-replicating machine that operates on a molecular level with the power to dismantle anything.
But there might be more enjoyable things than destroying civilizations and building empires. One interesting concept is the Matrioshka brain, a computer megastructure surrounding a star which can upload the consciousness of an entire civilization and ensure that its citizens experience a perfect life without ever feeling sad or bored. If built around a red dwarf such a computer could be powered for up to trillions of years. Aliens might be content not to explore the universe because they are inside such a Matrioshka brain, or we might be, without knowing it.
Finally, it is possible that there are other intelligent civilizations out there, but they are simply not interested in communicating with us. They may have their own reasons for keeping quiet, or they may not have the technology or inclination to send out signals that we can detect.
In conclusion there are many potential explanations for the Fermi Paradox, ranging from the possibility of self-destruction to galactic civilizations that destroy any civilization with a self-replicating machine. Unfortunately, several the solutions to the Fermi paradox come up against one important problem: we do not know how advanced current technologies are. We could be close to the limit of what is possible, or nowhere near it.