The Milky Way galaxy has two hundred billion stars and perhaps one hundred billion planets. If even a small fraction of those planets harbored life, and even if only a pathetic scattering of those planets had lifeforms which became intelligent, our galaxy would be teeming with alien civilizations; some of whom would be either looking for us or discoverable for at least a little while.
The number of alien civilizations the galaxy should have can be determined by an equation, the Drake Equation, that turns the above factors into variables. When you plug them into the formula, you find that there should be at least 20 civilizations in our cosmic neighborhood.
This makes the fact that we have yet to find any other life in the cosmos almost shocking when you think about it. This seeming discord, between how many advanced civilizations ought to be in space and the lack of evidence for any, is known as the Fermi Paradox. It has lead to dozens of hypotheses and potential solutions over the last few decades.
Many of the solutions aim at one of the variables in the Drake Equation and try to make the supposed number of civilizations lower, so it is more reasonable for us to have not met anybody yet.
Some propose that life starting at all is rare, others suggest that the development of intelligence is the bottleneck. Others posit that most civilizations would live for a short time before blowing themselves up or, conversely, never even manage to invent the radio.
The Dark Forest solution explains why we haven’t heard from aliens by positing that they are purposefully keeping quiet.
The reasoning is laid out best in the science fiction novel The Dark Forest, by Liu Cixin. The plot of the book, the second in a series, concerns questions of how to best interact with potentially hostile alien life.
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