The Milky Way galaxy has billions of planets that could potentially host life. Yet despite scientists' efforts to monitor for and occasionally signal to extraterrestrials, we have not found any evidence that aliens exist.

This conundrum is known as the Fermi Paradox, and it has inspired debate among researchers for decades.

A co-winner of this year's Nobel prize in physics said he "can't believe we are the only living entity in the universe," and that he's "absolutely convinced" we will detect alien life within 100 years.

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Didier Queloz, a physicist from the University of Cambridge, was named a co-winner of the Nobel prize in physics on Tuesday for his discovery of the first exoplanet orbiting a sun-like star.

Queloz, who calls himself a "planet hunter," said following the award ceremony that his work has led him to become "absolutely convinced" that humans will detect alien life in the next 100 years.

"I can't believe we are the only living entity in the whole universe. There's just way too many planets, way too may stars ... the chemistry that led to life has to happen elsewhere," Queloz in a talk at the Science Media Center in London on Tuesday.

His statement is, in a way, a response to a question that physicist Enrico Fermi first posed to his colleagues over lunch in 1950: "Where is everybody?"

Arguably, Fermi said, in the 4.4 billion years it took for intelligent life to evolve on our planet, the rest of our galaxy should have been overrun with similarly smart, technologically advanced aliens. But scientists have been monitoring radio waves for signs of alien life in the universe for decades, and they haven't found anything or anyone.

This conundrum has come to be known as the Fermi Paradox.

Scientists have offered myriad potential answers to Fermi's question, including that aliens are hibernating or deliberating hiding from us. Some researchers have also suggested that highly advanced technological civilizations destroy themselves before they have the opportunity to get in contact with other intelligent life in the universe.

Read More: Alien civilizations may have explored the galaxy and visited Earth already, a new study says. We just haven't seen them recently.

In his recent book "End Times," science writer Bryan Walsh discusses 13 theories as to why we've yet to make contact with aliens and why we might never do so. Here's how each one addresses the Fermi Paradox.