The best answer imho to the most abused citation by Carl Sagan on this topic.

I wanted to share that with you all, and it´s from an article entitled “The Solipsist Approach to Extraterrestrial Intelligence” by William Isaac Newman, and C. Sagan (the irony) in which is stated:

<<the absence of evidence is not the evidence of absence>>

This was a rebuttal of the Hart-Tipler Conjecture, which stated that the absence of evidence can only be explained by the absence of Extra Terrestrial Civilizations.

link to the article for reference:

https://www.researchgate.net/publication/234209662_The_Solipsist_Approach_to_Extraterrestrial_Intelligence