What is the Large Hadron Collider for?
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.
Researchers at Cern in Switzerland have already discovered the Higgs boson, and now hope to find out more about dark matter.