China has begun expanding the world's deepest underground lab in southwestern Sichuan Province, where scientists have been conducting experiments on mysterious "dark matter."

The second-phase construction of the Jinping Underground Laboratory, located at 2,400 meters under the surface of Jinping Hydropower Station, was launched on August 1 by Tsinghua University and Yalong River Hydropower Development Co.

The construction, scheduled to be completed by the end of 2015, will increase the lab's space to 120,000 cubic meters, allowing more experiments to be carried out simultaneously, the university said.

The Jinping lab, opened in December 2010, provides a "clean" space for scientists to pursue the invisible substance known as dark matter. Researchers said the extreme depth helps block most cosmic rays that can hinder observation.

The lab has accommodated a project called the China Dark Matter Experiment, whose results were published in scientific journal Physical Review in 2013.

Scientists are still searching for evidence to prove the existence of the hypothetical dark matter, which accounts for over a quarter of the universe's mass-energy balance but has not been directly detected.