When 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 percent 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 gravit