The world's biggest particle accelerator is back in action after a two-year shutdown and upgrade, embarking on a new mission that scientists hope could give them a look into the unseen dark universe.

Scientists at the European Organization for Nuclear Research, or CERN, on Sunday shot two particle beams through the Large Hadron Collider's 27-kilometre tunnel, beneath the Swiss-French border near Geneva.

CERN wrote on its website that "the startup is complete!"

A technician stands near equipment of the Compact Muon Solenoid (CMS) detector at the LHC particle collider. The machine had been shut down for two years for a refit. (REUTERS)

The collider was instrumental in the discovery of the Higgs boson, a subatomic particle that had long been theorized but never confirmed until 2013.

Scientists are promising nearly twice the energy and more violent particle crashes this time around, starting as early as June. They hope for a first ever glimpse of dark matter, one of the chief focuses of the experiment.

Dark matter — and its cousin, dark energy — make up most of the universe, but scientists haven't been able to see them yet, so researchers are looking for them in high-energy crashes, in orbit in a special experiment on the international space station, and in a deep underground mine.

CERN spent about $150 million on the upgrade, opening the massive machine every 20 metres, checking magnets and improving connections.

"After two years of effort, the LHC is in great shape," said CERN director for accelerators and technology Frédérick Bordry.

"But the most important step is still to come when we increase the energy of the beams to new record levels," he said.