hen it comes to understanding the fabric of the universe, most of what scientists think exists is consigned to a dark, murky domain. Ordinary matter, the stuff we can see and touch, accounts for just 5% of the cosmos. The rest, cosmologists say, is dark energy and dark matter, mysterious substances that are labeled “dark” partly to reflect our ignorance about their true nature.
While no single idea is likely to explain everything we hope to know about the cosmos, an idea introduced two years ago could answer a few big questions. Called the dark dimension scenario, it offers a specific recipe for dark matter, and it suggests an intimate connection between dark matter and dark energy. The scenario might also tell us why gravity — which sculpts the universe on the largest scales — is so weak compared to the other forces.
The scenario proposes an as-yet-unseen dimension that lives within the already complex realm of string theory, which attempts to unify quantum mechanics and Einstein’s theory of gravity. In addition to the four familiar dimensions — three infinitely large spatial dimensions plus one of time — string theory suggests that there are six exceedingly tiny spatial dimensions.
In the dark dimension’s universe, one of those extra dimensions is significantly larger than the others. Instead of being 100 million trillion times smaller than the diameter of a proton, it measures about 1 micron across — minute by everyday standards, but enormous compared to the others. Massive particles that carry the gravitational force are generated within this dark dimension, and they make up the dark matter that scientists think comprises about 25% of our universe and forms the glue that keeps galaxies together. (Current estimates hold that the remaining 70% consists of dark energy, which is driving the universe’s expansion.)
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,” said Ignatios Antoniadis, a physicist at Sorbonne University who is actively investigating the dark dimension proposal.
While there’s no evidence yet that the dark dimension exists, the scenario does make testable predictions for both cosmological observations and tabletop physics. That means we may not have to wait long to see whether the hypothesis will bear up under empirical scrutiny — or be relegated to the list of tantalizing ideas that never fulfilled their original promise.
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