In the 400 years since Galileo Galilei first held a telescope to the heavens, astronomers have laid bare some of the deepest mysteries of the cosmos. They have seen comets crash into planets, found oceans inside moons, and witnessed the shudder of spacetime as black holes collide.

But space remains a realm of the unknown. Writing in the journal Nature on Thursday, scientists in Canada reported the detection of mysterious radio signals from halfway across the universe. It is only the second time that repeating fast radio bursts, or FRBs, have been spotted.

Astronomers have yet to formulate a full theory of what produces these enigmatic, rapid-fire beams of electromagnetic waves. And in the absence of a firm explanation, speculation has fallen, perhaps inevitably, on alien civilisations. Avi Loeb, a Harvard astronomer, has proposed that FRBs might be powerful energy beams used to propel alien spacecraft.

It is not the first time that poorly-understood cosmic phenomena have been ascribed to industrious extraterrestrials. When in 2015 astronomers noticed a star, 1,500 light years distant, dimming and brightening, researchers suggested an “alien megastructure” might be revolving around it, and collecting energy for its constructors. Then, in 2017, the massive cigar-shaped ‘Oumuamua barrelled into the solar system, the first interstellar object known to do so, and prompted speculation that it was a tumbling spacecraft.

Chris French, head of the Anomalistic Psychology Research Unit at Goldsmiths, University of London, said it was natural for humans to see aliens behind every cosmic mystery. “We have what is called an intentionality bias,” he said. “It’s the assumption that whenever something happens, something or someone made it happen for a reason. In the context of space, that someone is always going to be aliens.”

The evolutionary argument for intentionality bias, or “agenticity”, is that our ancient ancestors fared better if a rustle in the bushes made them run for cover rather than assume it was the wind. “At the end of the day, our brains evolved to keep us alive rather than apprehend the truth of the universe,” said French.

The late astronomer Carl Sagan spotted the dilemma for scientists. They can become cranks if they are too open-minded, but may miss out on landmark discoveries if they are not open-minded enough. “It seems to me what is called for is an exquisite balance between two conflicting needs,” he said. “The most sceptical scrutiny of all hypotheses that are served up to us and at the same time a great openness to new ideas.”

Duncan Lorimer, an astrophysicist at West Virginia University, discovered the first FRB in 2007. When his team spotted the radio burst, the possibility of it being a message from ET certainly came up. “We absolutely thought about aliens,” he said. “We only had the one object. We looked for patterns in the signal and couldn’t find anything, but we definitely considered it.”

Now, mention of aliens is beginning to wear thin. “It helps to sell the story, but at this point I do roll my eyes a bit,” he said. Many astronomers now favour the idea that FRBs are intense beams of radiation shed by charged particles as they are whipped around by strongly magnetised neutron stars. “It seems like a more plausible idea, but I don’t want to rule out aliens completely,” he said. “I’ve been wrong before.”