Researchers from around the world are racing to find evidence of a mysterious substance believed to make up the vast majority of the universe.
More than 30 research projects are under way to find evidence of dark matter, which is so different from ordinary matter that it has never been detected.
Scientists have working more than 1500 metres underground in the hope of making a breakthrough.
Al Jazeera's Rob Reynolds reports from Deadwood, South Dakota.
Source: Al Jazeera