In 1950, physicist Enrico Fermi was having lunch with colleagues at Los Alamos. The conversation turned to flying saucers and extraterrestrial civilizations. Fermi suddenly asked: 'So where is everybody?' This simple question became one of the deepest paradoxes in science.
Our Galaxy contains 100–400 billion stars. The observable universe holds about 2 trillion galaxies. The universe is 13.8 billion years old, while Earth is only 4.5 billion. Even if intelligent life is extremely rare, statistically there should be thousands, if not millions, of civilizations.
Moreover, a civilization with technology slightly more advanced than ours could colonize the entire Galaxy in 1–10 million years — an instant in cosmic terms. Yet we see no traces: no radio signals, no megastructures, no probes. The silence of the cosmos — that is the Fermi Paradox.