Could Relic Black Holes from Past Universes Explain Dark Matter?

Dark Matter May Be Made of Black Holes from Dead Universes, New Theory Suggests

Dark matter—the invisible substance that shapes galaxies—might be composed of "relic" black holes that survived the collapse of previous universes, according to a new study published in Physical Review D. The research, led by physicist Enrique Gaztanaga, explores a radical idea: the universe may have undergone an endless cycle of expansion and contraction, with black holes from past epochs persisting into our current cosmos.

Gaztanaga outlines this theory in an essay for The Conversation, framing it around two key concepts: the "Big Bounce"—a controversial alternative to the Big Bang—and primordial black holes.

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

Why Black Holes Make a Strong Dark Matter Candidate

Black holes are an appealing candidate for dark matter because both are invisible yet exert gravitational influence. Unlike hypothetical particles or untested physics, black holes are already known to exist. The theory focuses on primordial black holes, which formed fractions of a second after the Big Bang—not from dying stars, but from the extreme density of the early universe.

These primordial black holes could theoretically range from microscopic to massive, yet none have been directly detected. Gaztanaga’s model suggests some may be remnants of a prior universe that collapsed into a singularity before "bouncing" into our current expansion phase.

How Relic Black Holes Could Survive Cosmic Collapse

In Gaztanaga’s framework, black holes larger than 90 meters in size could have endured the transition from a contracting universe to an expanding one. These "relics" would carry traces of the previous cosmic epoch, including black holes, gravitational waves, and density fluctuations.

Relic black holes could form in two ways: by surviving the collapse directly or by condensing from matter during the bounce. Gaztanaga explains that entire galaxies and stars from the prior universe might have collapsed into black holes, "erasing most of their detailed structure but preserving their mass."

Could These Black Holes Explain Dark Matter and Early Supermassive Black Holes?

If the Big Bounce produced enough relic black holes, they could account for a significant—or even dominant—portion of dark matter. This theory also offers a potential explanation for the existence of supermassive black holes observed just hundreds of millions of years after the Big Bang, which conventional models struggle to account for due to their rapid growth requirements.

However, Gaztanaga emphasizes that the idea remains speculative. "Much work remains to be done," he notes, as the hypothesis must be rigorously tested against observational data, including gravitational wave backgrounds, galaxy surveys, and cosmic microwave background measurements.

What’s Next for This Radical Cosmological Idea?

The study joins a growing body of research exploring unconventional explanations for dark matter and the early universe. While the Big Bounce and relic black holes present compelling possibilities, they require further empirical validation before gaining widespread acceptance in the scientific community.