Robert H. Gray
In 2016, Gray published an article in "Scientific American" about the Fermi paradox, which claims that if extraterrestrials existed, we would see signs of them on Earth, because they would certainly colonize the galaxy by interstellar travel. Gray argues that the Fermi paradox, named after Nobel Prize-winning physicist Enrico Fermi, does not accurately represent Fermi's views. Gray states that Fermi questioned the feasibility of interstellar travel, but did not say definitively whether or not he thought extraterrestrials exist.
Fermi paradox
It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on percolation theory. Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters," with large areas remaining uncolonized at any one time.