The Fermi Paradox is described as; The apparent contradiction between high estimates of the probability of emergence of life (Particularly intelligent life) in the universe and the apparent lack of it. Since a technological civilization capable of ISRU and self-replication could occupy a galaxy in less than a million years (See Von Neumann Probes), and Kardaschev >= 2 civilizations can be identified by their waste heat, the apparent lack of intelligence implies, either, that other intelligent life does not exist, or that there is a "Great Filter" that prevents technological civilization from spreading into interstellar space.
Some wonder if the fermi paradox can also be explained by what is referred to as "Zoo hypothesis", whereby humans are said to be observed by other intelligent beings in the universe. The opposite of this theory being self-imposed quarantine. 
Posthuman intelligence theories
NSA whistle blower Edward Snowden has theorized that we may never detect alien life due to their use of encryption and obscuration of their communications. Others have speculated that alien life is most likely AI-based or otherwise digital in nature, rather than fleshy green or gray aliens typically portrayed in fiction.