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Thanks to gravity, we at least know that dark matter exists. We also know that it's frighteningly abundant, making up about 85 percent of all matter in the universe. However, apart from this, we know almost nothing. We don't know what dark matter is or where it came from, and if it consists of some weakly interacting form of matter, we still can't directly detect a single particle of it.
According to a theoretical study by two researchers from Colgate University in the US, our continued failure to detect dark matter - even with the latest, highly sensitive detection methods - calls for a rethinking of the nature and potential origin of this mysterious substance.
Instead of emerging from the Big Bang along with normal baryonic matter, dark matter may have emerged a bit later in its own "Dark Big Bang," the study authors write. It can now inhabit the dark sector, mostly separated from our visible sector, interacting with it solely through gravity.
Gravity taught us how little we still know about dark matter. We call this ghostly substance dark matter because it does not interact with light or other electromagnetic radiation, making it invisible to us.
However, given its gravitational effect on galaxies and galaxy clusters, as well as other evidence such as its effect on the cosmic background radiation, we have strong evidence that dark matter exists. We just still know next to nothing about everything else, including what it is and where it came from.
The hunt for dark matter has largely focused on Weakly Interacting Massive Particles, or WIMPs. These hypothetical elementary particles interact via two of the four fundamental forces: gravity and the weak nuclear force (but not electromagnetism or the strong nuclear force).
Decades of searching have not yielded any confirmation of the existence of wimps, which somewhat reduced their appeal and supplanted scientific interest in other possible explanations for dark matter.
"As WIMPs continue to elude detection, it becomes increasingly important to look at the dark sectors, which are strongly separated from the visible sector," write the authors of the new study, physicist Cosmin Illier and graduate student Richard Casey.
In 2023, two other researchers—Katherine Freese and Martin Winkler of the University of Texas at Austin—proposed an intriguing theory: Dark matter may have formed separately from the initial cosmic expansion in a second big bang, which they called the Dark Big Bang.
This challenges the way many scientists imagine this pivotal epoch when all matter and radiation in the universe (including dark matter, whatever it is) comes from a single sector of physics.
However, according to Freese and Winkler, the dark Big Bang is consistent with evidence for the existence of dark matter—provided it happened quickly, within about a month of the first Big Bang.
In this scenario, dark matter particles arise from the decay of a quantum field that interacts only with the dark or hidden sector—a hypothetical set of particles and forces beyond our current knowledge of physics.
The Dark Big Bang would cause a first-order cosmological phase transition in the dark sector, comparable to the change in reality in the visible sector after the first Big Bang, which transformed vacuum energy into a hot plasma of radiation and particles.
In their new study, Ilieu and Casey dig deeper into this idea, examining its feasibility and testing a number of different dark big bang scenarios that are consistent with existing experimental data. Their work helps to strengthen the argument of Freese and Winkler, not only confirming the possibility of a Dark Big Bang, but also comprehensively evaluating various options for the development of such an event.
Among their findings, Ilieu and Casey reveal a previously unexplored set of potential parameters for dark matter nucleation. They also suggest options for further testing this concept, including the possible existence of detectable traces, such as gravitational waves, left behind by these scenarios.
"The detection of gravitational waves generated by the dark Big Bang could provide crucial evidence for a new theory of dark matter," says Illier.
And while dark matter is unlikely to give up its secrets easily, there is reason to be optimistic that we will solve this cosmic mystery, he notes.
"With modern experiments like the International Pulsar Timing Array (IPTA) and the Square Kilometer Array (SKA) on the horizon, we may soon have the tools to test this model in unprecedented ways," says Illier.
For example, in 2023, researchers from the NANOGrav research collaboration, which is part of IPTA, announced that they had found evidence for the existence of a gravitational-wave background in the universe. This could help test the dark Big Bang concept, Illier and Casey say, setting the stage for further research that could finally shed some light on dark matter. The study was published in the journal Physical Review D.
