Dark matter: search for the invisible begins in an old gold mine

Scientists know that it makes up most of the universe's mass, but they don't know what it is ... or exactly how to find it

In a former gold mine a mile underground, inside a titanium tank filled with a rare liquified gas, scientists have begun the search for what so far has been unfindable: dark matter.

Scientists are pretty sure the invisible stuff makes up most of the universe's mass and say we wouldn't be here without it - but they don't know what it is. The race to solve this enormous mystery has brought one team to the depths under Lead, South Dakota.