There could be a simple reason why the universe shows signs of alien life while astronomers have struggled to find concrete evidence, the aliens are all gone.

A pair of scientists at the Australian National University posed this possibility in a new study published in the journal Astrobiology. Collaborating on the paper was Aditya Chopra, of ANU's Research School of Earth Sciences, and Charley Lineweaver, of the school's Planetary Science Institute.

"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," Chopra said in a press release. "Early life is fragile, so we believe it rarely evolves quickly enough to survive.

"Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable."

The researchers suggested Venus and Mars were habitable four billion years ago, as was Earth. The problem Venus and Mars faced was one the Earth handled, and thus sustained microbial life: the ability to "stabilize the rapidly changing environment."

In 1950, Enrico Fermi noted how "habitable" exoplanets seem to be all over the known universe yet no evidence of past microbial life has been found to date. Even with various technological advances, that still holds true today, thus Fermi's Paradox.

Chopra and Linewater dubbed their theory the Gaian Bottleneck and they believe it explains Fermi's Paradox.

"Life on Earth probably played a leading role in stabilising the planet's climate," Chopra said. "The mystery of why we haven't yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces."