Summarizing the Paradox
The Fermi Paradox, which is sometimes referred to as the Fermi-Hart Paradox, is an intellectual exercise addressing the possibilities of advanced, intelligent life in the universe. Simply put it states the since our solar system is fairly young there should be much older planetary systems that support life, some may have intelligent, advanced species possibly a billion years older than Earth. If this is the case and they have discovered interstellar travel then the entire galaxy could be colonized in less than 100 million years. The Earth, therefore, should have been colonized or at least visited and yet there is no credible evidence of contact which leads one to ask where are the advanced civilizations? The paradox is that all the available evidence (such as the size of the universe, its age, the relatively common occurrence of the elements believed to be necessary for life) suggests that there should be multiple advanced alien civilizations and yet
there is no evidence such civilizations exist.