CERN's Large Hadron Collider will be turned back on in March and a few weeks later will start smashing sub-atomic particles together again at nearly double its previous power, helping scientists hunt for clues about the universe.

The world's biggest particle collider, located near Geneva, has been undergoing a two-year refit and work is now "in full swing" to start circulating proton beams again in March, with the first collisions due by May, the European Organization for Nuclear Research (CERN) said on Friday.

"With this new energy level, the (collider) will open new horizons for physics and for future discoveries," CERN Director General Rolf Heuer said in a statement. "I'm looking forward to seeing what nature has in store for us."

CERN's collider is buried in a 27-kilometre tunnel straddling the Franco-Swiss border at the foot of the Jura mountains. The entire machine is already almost cooled to 1.9 degrees above absolute zero in preparation for the next three-year run.

Higgs boson particle confirmed in 1st run

The first run, carried out at lower power, led in 2012 to confirmation of the existence of the Higgs boson particle, which explains how fundamental matter took on the mass to form stars and planets.

That discovery was a landmark in physics but there are still plenty of other mysteries to be unravelled, including the nature of "dark matter" and "dark energy".

We have unfinished business with understanding the universe. - Tara Shears, University of Liverpool physics professor

Latest calculations suggest that dark matter accounts for 27 per cent of the universe and dark energy, which drives galaxies apart, 68 per cent, while the visible matter observed in galaxies, stars and planets makes up just five per cent.

Other unsolved questions include the relative lack of antimatter in the universe, when equal amounts of matter and antimatter were created in the Big Bang 13.8 billion years ago, and the possible existence of other new kinds of particles.

Many physicists favour a yet-to-be-proven theory known as super-symmetry, in which all basic particles have a heavier but invisible "super" partner.

Getting to grips with such issues requires deeper insights into the building blocks of the cosmos, which researchers hope to achieve by turning up the dial at CERN to higher energies.

"We have unfinished business with understanding the universe," said Tara Shears, a physics professor at the University of Liverpool, who works on one of the four main experiments at the collider.