Here’s a fun dark matter solution. The invisible and as yet hypothetical substance could be made of “relic” black holes that survived the deaths of previous universes.

This idea is explored as part of a new study published in the journal Physical Review D, which the lead author Enrique Gaztanaga breaks down in an essay for The Conversation. It hinges on two key concepts: the “Big Bounce,” a controversial idea that the Big Bang was part of an endless cycle in which the universe expands and contracts and expands again; and so-called primordial black holes.

“The universe may not have begun once, but may have rebounded,” Gaztanaga wrote. “And the dark structures shaping galaxies today could be relics from a time before the Big Bang.”

Black holes as a dark matter candidate have a powerful allure. Both are largely invisible but still massive, and pinning the blame on black holes means we don’t have to search for undiscovered particles or an entirely new form of physics. 

Such theories focus on primordial black holes, which are thought to have formed just seconds after the Big Bang. Whereas typical black holes are born from the explosive death of a star, primordial ones spawned directly from the extreme conditions of the early cosmos, potentially starting as small as an atom. These should be swarming across the universe, but none have been detected.

Still, many physicists believe they’re out there. In Gaztanaga’s model, some of them may be relics from a previous universe that contracted in on itself and destroyed everything as it compressed into a singularity. Or almost everything.

“In our work, we found that things larger than 90 metres could have survived the transition from collapse to expansion,” Gaztanaga said. “This leaves behind ‘relics’ that carry information from a previous cosmic epoch. These relics can include black holes, gravitational waves and density fluctuations.”

In this model, relic black holes can arise either through direct survival, or by forming from clumps of matter that collapse into the highly compact objects. In effect, galaxies and stars from the contraction phase become black holes, Gaztanaga explained, “erasing most of their detailed structure but preserving their mass.”

The possibilities are tantalizing. If the bounce produces enough relic black holes, “they could make up a significant — perhaps dominant — fraction of dark matter.” They could also explain how we’re finding supermassive black holes that existed just hundreds of millions of years after the Big Bang, which shouldn’t be possible because they need more time to accrue their unbelievable mass.

But “much work remains to be done,” cautions Gaztanaga, as the “ideas must be tested against data — from gravitational-wave backgrounds to galaxy surveys and precision measurements of the cosmic microwave background.”

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