Astronomers detect for the first time dark matter dangling from cosmic web

For the first time, astronomers have identified dark matter suspended from expansive filaments that span the universe, creating a "cosmic web" that ensnares galaxies much like morning dew on a spiderweb. Researchers from Yonsei University in Seoul, South Korea, utilized the Subaru Telescope—an 8.2-meter optical-infrared telescope near the summit of Maunakea in Hawaii—and exploited the gravitational effect on light to indirectly observe dark matter positioned on the cosmic web filaments within the Coma Cluster.

This groundbreaking discovery represents the initial detection of dark matter within the cosmic web and has the potential to provide insights into how this vast structure, featuring strands extending over tens of millions of light-years, has influenced the evolution of the universe. Known as Abell 1656, the Coma Cluster comprises over a thousand galaxies and lies approximately 321 million light-years away in the direction of the constellation Coma Berenices. Due to its considerable size and relative proximity, the Coma Cluster serves as an optimal location for scientists to investigate dark matter along cosmic web strands.

The cosmic web, an intricate network of filaments composed of matter, facilitates the nourishment of gas into galaxies, promoting their growth. Additionally, it plays a crucial role in guiding galaxies together, encouraging clustering. The primary filaments of the cosmic web constitute the walls of galaxy superclusters, with the Coma Cluster's corresponding wall recognized as the "great wall," the inaugural superlarge structure discovered in the universe.

Clusters of galaxies are believed to congregate at intersections of filaments, and these filaments are thought to conclude between galaxies, forming what is termed "intracluster filaments." Dark matter is anticipated to traverse along these cosmic web filaments, hanging from these intracluster filaments.

Dark matter as a cosmic scaffold

Regarding the cosmic web, the universe's largest structure, its existence has been acknowledged for decades, but astronomers have only observed the faint glow of its gas filaments when illuminated by bright regions at the cores of galaxies powered by active supermassive black holes, known as quasars.

In the previous year, the Keck Cosmic Web Imager, situated atop Maunakea, captured the initial direct light emitted by delicate web-like filaments intersecting and extending through the most obscure realms of space. These filaments exist in isolation between galaxies, occupying the vast and concealed expanses of the cosmic web.

However, discerning the location of dark matter surrounding these cosmic web strands presents a distinct challenge. Despite constituting an estimated 85% of the universe's matter, dark matter remains invisible as it lacks interaction with light, unlike everyday matter composing stars and dust. The prevalence of dark matter over everyday matter extends to the domination of cosmic web filaments, forming an imperceptible scaffold that shapes the structure of the universe.

Despite its non-interaction with light, dark matter does interact with gravity, influencing the movement of observable matter and light. Leveraging this concept, the research team utilized the Subaru telescope's Hyper Suprime-Cam (HSC) with high sensitivity, resolution, and a wide field of view to detect the weak lensing effect of the dark matter component on intracluster filaments within the Coma Cluster. This approach involved observing the gravitational lensing phenomenon, as predicted by Albert Einstein's general relativity theory from 1915, where the curvature of spacetime, caused by mass, diverts the path of light from a background source.

By applying this gravitational lensing technique to light from galaxies and stars situated behind the Coma Cluster, the team achieved the groundbreaking feat of identifying the weak lensing effect of dark matter on the terminal segments of the cosmic web. Published in the journal Nature Astronomy in January, this unprecedented detection contributes to reinforcing the confirmation of the extensive large-scale structure permeating the universe.