Scientists have successfully restarted the Large Hadron Collider (LHC), the most powerful atom-smasher ever built, hoping to enter a new realm of physics and make history for the second time.

Two beams of particles travelling a whisker below the speed of light were sent flying in opposite directions through the LHC's 27 kilometres (16.7 miles) of circular underground tunnels straddling the Swiss-French border.

Amid scenes of jubilation in the LHC control room, Professor Rolf Heuer, director general of Cern, the European Organisation for Nuclear Research, said: "Congratulations. Thank you very much everyone... now the hard work starts."

Currently the £3.74 billion machine is running at a low "injection" energy of 450 giga-electron volts (GeV). In June, the energy level will be ramped up to a record-breaking 13 tera-electron volts (TeV) and experiments probing the fundamental building blocks of the universe can begin.

Two years ago the LHC team, which includes a number of British physicists, astounded the world with the discovery of the Higgs boson, an elementary particle that gives other particles mass.

Now the scientists have their sights set on an even more exotic trophy - dark matter, the invisible, undetectable material that makes up 84% of matter in the universe and binds galaxies together yet whose nature is unknown.

With a beam energy of 13 TeV - almost twice that which produced the Higgs boson - it is conceivable that the LHC will capture dark matter, marking a leap forward in our understanding of the universe.

Earlier Cern spokesman Arnaud Marsollier said: "The LHC will be running day and night. When we will get results we don't know. What is important is that we will have collisions at energies we've never had before.

"If something interesting appears in this new window we will see it. It might be two months from now or two years, we're not able to say. It took 50 years to find the Higgs boson and 20 years to build this machine, and it will be running at least until 2035, so we can be patient."

A technical hitch had delayed the restart of the LHC after a two year re-fit and upgrade.

An electrical short circuit prompted fears that operations could be put back weeks or even months. However, engineers quickly located the problem - a small piece of metal debris - and removed it.

As tension built during the final minutes before the restart, Frederick Bordry, Cern's director for accelerators and technology, handed out Easter eggs to staff in the LHC control room.

The particles of protons, the "hearts" of atoms, travel round the LHC at just three metres per second below the speed of light.

When experiments begin they will be smashed together in four giant detectors located around the beam ring, sparking the creation of new particles and hopefully opening up a new era in physics.

The search for dark matter involves stepping outside the Standard Model, the all-encompassing theory that describes the particles and forces of nature that has stood firm for the past 50 years.

A "new physics" model of the universe called supersymmetry predicts that every known particle has a more massive partner - and one of these elusive supersymmetry particles might be the source of dark matter.

At a Cern briefing in Geneva last month, British scientist Professor David Charlton, from the University of Birmingham, who heads the Atlas detector team, said: "We're heading for unexplored territory. It's going to be a new era for science."

Professor Jonathan Butterworth, from University College London, a leading member of the Atlas detector team, said: "It all seemed to go very well this morning. We'll all be watching very excitedly to see what develops over the next few weeks. The LHC's operating again for the first time in two years and that's a really important milestone in physics.

"You can think of the LHC as the world's greatest microscope looking into the heart of matter. At this higher energy level we don't know what we'll see - no-one has looked there before.

"The next milestone will be to ramp up the energy because what we're really interested in is the collisions. Where we are at the moment, we've found the Higgs boson but there are obvious things we haven't been able to describe and we'd like some clues.

"It would be absolutely lovely to find a candidate for dark matter. That's a fairly glaring hole in our theory. Supersymmetry particles are a good candidate, but they're not the only candidate.

"The fact that we don't know what most of the universe is made off is really what drives me on."

Professor Charlton said a couple of "splash events" - when the beam is directed at a test target - confirmed that the particles had arrived at the Atlas detector.

He said: "It was a happy moment in the Atlas experiment control room when we saw the first beam splash event.

"This is a first step towards the exciting physics we will do in the coming, second, multi-year run of the LHC, which should give us new insights into fundamental physics, when we have collected plenty of collisions at the new high energies to come. The Atlas experiment, along with the other experiments at the LHC, is ready and eagerly waiting."

The three other LHC detectors are called CMS, Alice and LHCb.