Below the mountains in southwest China, the world's deepest and largest underground laboratory has just opened. This vast site is home to scientists dedicated to hunting down dark matter.

(Photo: Wikimedia Commons/ 刘二少)

Challenges in Detecting Dark Matter

In the vast universe, visible matter varies in size from a dust grain to a planet or nebula. No matter how massive they seem, they only account for about 5% of the universe's total mass, the remaining 95% being made of dark matter and dark energy.

Dark matter is the hypothetical substance that could be responsible for the organization of galaxies on grand scales. It is an invisible component of the universe whose presence is discerned only from its gravitational attraction rather than its luminosity.

It is tough to detect dark matter directly because there is a very low probability that it will interact with visible matter. Aside from this, ubiquitous cosmic rays also greatly interfere with the efforts of physicists to detect dark matter.

Because of this, efforts to detect dark matter must be accomplished with a laboratory that can shield cosmic rays and provide a research environment and facilities with a shallow radiation background. The US, Japan, and other European countries have built underground laboratories that meet this requirement.

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Record-Breaking Research Facility

In 2009, Tsinghua University and the Yalong River Hydropower Development Company, Ltd. began constructing the first phase of the China Jinping Underground Laboratory (CJPL). It was about 1.5 miles (2,400 meters) under the Jinping Mountain in Sichuan's Liangshan Yi Autonomous Prefecture.

The first-generation efforts, called CJPL-I, were completed and put into use at the end of 2010. It features a room capacity of almost 141,258 cubic feet (4,000 cubic meters). Because of its thick wall, the laboratory is exposed to only a tiny flux of cosmic rays, which is only a hundred millionth of what is found on the surface. This project also elevated the country's dark matter direct detection experiments to an advanced level on the global stage.

Due to the growing list of achievements in the first phase of CJPL, the authorities thought that the researchers needed more space to conduct further studies. In 2014, the Yalong River Hydropower Development Company, Ltd. and Tsinghua University agreed to build a second phase with an expanded room capacity of about 11.6 million cubic feet (330,000 cubic meters).

The project's second phase, the Deep Underground and Ultra-low Radiation Background Facility for Frontier Physics Experiments (DURF) is hailed as the largest and deepest ultra-clean subterranean space for scientific research. A total of 10 teams from various universities and research institutions are stationed there to unveil one of the universe's greatest mysteries.

The facility's construction started in December 2020 and concluded in December 2023. It surpassed the previous record for the most profound and extensive underground research laboratory held by the Laboratori Nazionali del Gran Sasso in Italy.

To ensure the cleanest possible environment for DURF, builders had to meet the requirements of extremely low environmental radiation, ultra-low cosmic ray flux, ultra-clean space, and deficient radon concentration. Aside from this, all materials and equipment must also have a background radiation of less than three times that of the radioactivity levels of rocks surrounding the cavern.

As China's major national project, the facility is expected to develop into a world-class platform integrating multiple disciplines such as nuclear astrophysics, particle physics, and life sciences.

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Check out more news and information on Dark Matter in Science Times.