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 16.8-mile tunnel, beneath the Swiss-French border near Geneva.

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. They hope to see all sorts of new physics, including a first ever glimpse of dark matter, during the collider's second three-year run.

CERN said the restart went smoothly and faster than expected. Still, it will be a while yet before the accelerator is working at full speed and particle crashes start.

"It will take us about six weeks to two months to establish the first stable collisions for the experiments, because we have to commission all the instruments, all the systems one by one," said Joerg Wenninger, the accelerator's coordinator of operations.

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 66 feet, checking magnets and improving connections.