Generally speaking, black holes are regions of spacetime where matter is compressed into a tiny space. Dark matter, meanwhile, is matter that does not reflect or absorb light. We know it exists because of its gravitational influence on galaxies and other cosmic structures.

It can be viewed as the "glue" that holds galaxies together, but we do not know what it is made of at a fundamental level. Most physicists think dark matter is composed of an as-yet-undiscovered sub-atomic particle.

But ancient black holes from before the big bang also fit the bill. They are dark, but also carry mass—exactly the properties required.

I have explored this idea in a new paper. Of course, the idea of relic black holes also requires a rethink of the big bang itself.

For nearly a century, cosmologists traced the history of the universe back to this single, dramatic moment. But maybe this wasn't the absolute beginning of time. Perhaps there was a universe before the big bang.

Under this scenario, the universe collapsed before undergoing an expansion. The big bang represents the transition between the two phases.