Fermi paradox
Michael H. Hart published in 1975 a detailed examination of the paradox, which has since become a theoretical reference point for much of the research into what is now sometimes known as the Fermi–Hart paradox. Geoffrey A. Landis prefers that name on the grounds that "while Fermi is credited with first asking the question, Hart was the first to do a rigorous analysis showing that the problem is not trivial, and also the first to publish his results". Robert H. Gray argues that the term "Fermi paradox" is a misnomer, since in his view it is neither a paradox nor due to Fermi; he instead prefers the name Hart–Tipler argument, acknowledging Michael Hart as its originator, but also the substantial contribution of Frank J. Tipler in extending Hart's arguments.
Fermi paradox
It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would be also very difficult for us to detect, short of a visit by a probe, a trip that would take hundreds of thousands of years with current technology.
To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of spaceflight and radio technology, lends more credence to the idea that technologically advanced civilizations are rare in the universe.