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The Large Hadron Collider was restarted today after being turned off for two years during a major renovation project which cost £100million.

The world's largest atom-smashing machine is most famous for proving the existence of the Higgs boson - but scientists hope it will now unlock even more fundamental secrets of the universe.

Physicists at Cern, the Geneva-based organisation which runs the LHC, are aiming to see dark matter for the first time ever thanks to the device's upgrade.

Back to work: The Large Hadron Collider is operating again after two years being upgraded

The LHC is situated underground below the border between Switzerland and France, and consists of nearly 17 miles of circular tunnels.

It was shut down two years ago - after confirming the discovery of the Higgs boson - so that its energy levels could be almost doubled, allowing scientists to carry out more extreme experiments.

The re-opening was delayed by another couple of weeks thanks to a short circuit in the system, but this morning it started operating again at last.

After two beams of particles were sent flying through the tunnels at a speed just slower than light, Cern's director general Rolf Heuer said: 'Congratulations. Thank you very much everyone... now the hard work starts.'

Workers - who were earlier given Easter eggs by one of the centre's bosses - cheered when it was confirmed that the machine was back in operation.

Ambition: Scientists hope the machine will be able to detect and describe dark matter for the first time

The LHC, which cost nearly £4billion, is currently running at a low 'injection' energy of 450 giga-electron volts (GeV), but in June it will increase to a record-breaking 13 tera-electron volts (TeV) - up from 7 TeV at the time it managed to detect the Higgs boson in 2013.

British scientist Peter Higgs was awarded the Nobel Prize after the discovery of the particle, which he and others predicted would exist but which had never been seen until the construction of the LHC.

Now the physicists operating the machine have their sights on dark matter, the invisible, undetectable material that makes up 84 per cent of matter in the universe and binds galaxies together yet whose nature is unknown.

If they are able to detect and describe dark matter, it will mark a huge leap forward in our understanding of the universe.

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.'

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 moving beyond the Standard Model, the all-encompassing theory describing the particles and forces of nature that has stood firm for the past 50 years.

Inside: The LHC consists of an enormous circular tunnel which is nearly 17 miles long

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. 

Professor Jonathan Butterworth, from University College London, 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.

'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 of is really what drives me on.'

As well as dark matter, the team is hoping to discover more Higgs bosons and other 'exotic particles' which have never been detected before.

They also believe they could uncover the existence of extra dimensions, which may explain why gravity is weaker than theoretical models would predict.

And the LHC could cast light on antimatter, particles which are identical to normal matter but with the opposite charge.

However, some have expressed concerns that the machine could be seriously unsafe, as its high-energy experiments create conditions similar to black holes which could have catastrophic consequences. 

Professor Andy Parker, head of physics at Cambridge University, said: 'This is a very exciting time for scientists as the Large Hadron Collider restarts after a major upgrade.

'The current Standard Model explains the known particles and forces, and the discovery of the Higgs completed that picture. But the Standard Model does not explain dark matter, which is believed to make up most of the universe, nor dark energy, a mysterious force driving the galaxies ever further apart.

'Interestingly, the answers to these problems in cosmology might lie in the realm of sub-atomic physics studied at Cern. The LHC might be able to produce dark matter particles for example, which would be glimpsed in the debris of collisions detected by the Atlas and CMS experiments.

'Even more exciting is the possibility that the universe could have more than three space dimensions, and that other spaces are hidden all around us.'

And Professor Dave Newbold, who heads the particle physics group at the University of Bristol, added: 'The discovery of the Higgs boson in 2012 was a huge breakthrough, but the real excitement starts now.

'The upgraded LHC will allow us to probe further than ever before, and to explore brand new scientific territory such as dark matter.'