The Drake Equation
The Drake equation is an attempt to estimate the probability of contacting intelligent life on other planets, or, more generally, the probability that intelligent life exists on other planets. It is expressed as a product of a bunch of probabilities all of which were largely unknown when it was formulated nearly fifty years ago. Some of the more important were the fraction of stars with planets, the fraction of planets that were Earth like, and the probability of life arising on an Earth like planet - all of which were squarely in the unknown category a half century ago. Many hundreds of planets have now been discovered around nearby stars, and a significant fraction of them are apparently Earth like in size at least.
One investigator suggested something like 100 million Earth like planets in the Galaxy - about one for every 1000 stars. The probability of life arising on such a planet is still unknown, but there are increasingly strong hints that it could be substantial. That still leaves the question of the probability of the development of technological civilization as an nearly total unknown, as is the urgent question of whether such civilizations inevitably destroy themselves and their planet.
If intelligent life is common in the Galaxy, we are left with Fermi's question: "where are they?" Of course if they are advanced enough to cross interstellar space, they are probably capable of being as inconspicuous as they wish.