LHC tunnel
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LHC restart could help with dark matter

5 April 2015 Last updated at 11:36 BST

The restart of the Large Hadron Collider (LHC) is now under way for the first time since 2013.

It's the world's biggest experiment and scientists hope will lead to a deeper understanding of how the universe works.

For the last two years, physicists have been working on an upgrade to the giant machine.

The LHC is a 16-mile-long tunnel, which sits 100m below ground on the border between France and Switzerland.

Tim O'Brien is a professor of astrophysics from the Jodrell Bank observatory. He says the most exciting discovery could be dark matter.