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.