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Why does the size and age of the Milky Way Galaxy make the Fermi paradox puzzling?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

The Fermi paradox highlights a contradiction between the high statistical probability of extraterrestrial intelligence and the total lack of evidence for it. The age and size of the Milky Way Galaxy make this particularly puzzling due to the vast time available for colonization.1

The Milky Way is approximately 100,000 light-years in diameter and roughly 10 billion years old. Given these dimensions, even a civilization traveling at a modest fraction of the speed of light—such as one-hundredth of its speed—could theoretically explore or colonize the entire galaxy in about 10 million years.12

Because Earth has existed for 4.5 billion years, there has been ample time for such a civilization to have traversed and settled the galaxy many times over. With billions of stars and a significant fraction likely hosting habitable planets, the absence of any signs of visitation or communication—often called the "Great Silence"—suggests that either the emergence of intelligent life is extremely rare, or that advanced civilizations face significant barriers to expansion or survival.1