BEIJING - China has begun expanding the world's deepest underground lab in Southwest China's 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 Friday by Tsinghua University and Yalong River Hydropower Development Company, the university told Xinhua on Saturday.

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 for scientists to pursue the invisible substance known as dark matter. Researchers said the extreme depth helps block most cosmic rays that mess with the observation.

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

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