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In a ‘Dark Dimension,’ Physicists look for the Cosmos’s Missing Matter

A notion derived from string theory suggests that dark matter is hiding in a relatively large extra dimension. The theory makes testable assumptions that physicists are examining now.

Most of what researchers think exists is consigned to a dark, murky domain, when it comes to understanding the fabric of the cosmos. Ordinary matter (the stuff we can see and touch) accounts for just 5% of the universe. The rest, cosmologists say, is dark matter and dark energy, mysterious substances that are branded “dark” partly to mirror our ignorance about their real nature.

While no lone idea is likely to explain everything we desire to know about the universe, a notion introduced two years ago could answer some big questions. Termed the dark dimension scenario, it offers a precise recipe for dark matter, and it suggests an intimate link between dark energy and dark matter. The scenario might also divulge to us why gravity — which shapes the universe on the largest scales — is so very weak compared to the other forces.

The scenario propounds an as-yet-unseen dimension that exists within the already complex realm of string theory, which tries to unify quantum mechanics and Einstein’s theory of gravity. Along with the four familiar dimensions — three infinitely big spatial dimensions plus one of time — string theory proposes that there are six extremely tiny spatial dimensions.

In the dark dimension’s universe, one of those additional dimensions is ominously larger than the others. Not 100 million trillion times smaller than the diameter of a proton, it measures around 1 micron across — miniscule by everyday standards, but massive compared to the others. Gigantic particles that carry the gravitational force are bred within this dark dimension, and they make up the dark matter that scientists believe comprises about quarter of our universe and forms the glue that keeps galaxies together. (Current estimations hold that the residual 70% consists of dark energy, which is driving expansion of the universe.)

The scenario “allows us to make connections between string theory, quantum gravity, particle physics and cosmology, [while] addressing some of the mysteries related to them,” stated Ignatios Antoniadis (a physicist at Sorbonne University), who is keenly investigating the dark dimension proposal.

While there’s no concrete evidence yet that the dark dimension exists, the scenario without any doubt makes testable predictions for both tabletop physics and cosmological observations. That means we will very soon get to know whether the hypothesis will hold up under empirical scrutiny — or be relegated to the long list of alluring ideas that never fulfilled their initial promise.

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