Scientists at the CERN research centre have discovered a new subatomic particle that could be the elusive Higgs boson.
The Higgs boson is thought to give matter mass and is believed to be crucial in the formation of the universe.
The team working at the Large Hadron Collider outside Geneva has spent years looking for evidence of the subatomic particle.
The Higgs particle is the last undiscovered piece of the Standard Model that describes the fundamental make-up of the universe.
It is a hugely successful theory but has long had several gaps, the biggest of which is why some particles have mass and others do not.
CERN Director General Rolf Heuer said: "We have reached a milestone in our understanding of nature.
"The discovery of a particle consistent with the Higgs boson opens the way to more detailed studies, requiring larger statistics, which will pin down the new particle's properties."
Mooted by British physicist Peter Higgs in 1964, the boson is believed to exist in an invisible, ubiquitous field created a trillionth of a second after the Big Bang, 13.7bn years ago.
Bosons are non-matter particles which are force carriers, or messengers that act between matter particles.
The interaction gives rise to three fundamental forces - the strong force, the weak force and the electromagnatic force. There is a fourth force, gravity, which is suspected to be caused by a still-to-be found boson named the graviton.
When some particles encounter the Higgs boson, they slow down and acquire mass, according to theory. Others, such as particles of light - or photons - are impervious to it and have no mass.
Prof Higgs expressed joy at the latest likely discovery.
"I never expected this to happen in my lifetime and shall be asking my family to put some champagne in the fridge."
Five sigma discovery
CERN uses a giant underground laboratory - the Large Hadron Collider - in order to search for the particle. Protons are smashed together at nearly the speed of light, yielding sub-atomic debris which is then scrutinised for signs of the fleeting Higgs.
The task is arduous because there are trillions of signals, occurring among particles at different ranges of mass.
Over the years, tens of thousands of physicists and billions of euro have been thrown into the search for the Higgs, gradually narrowing down the mass range where it might exist.
Two CERN laboratories - CMS and Atlas - worked independently of each other to avoid bias. They both found the new particle in the mass region of around 125-126 Gigaelectronvolts (GeV).
The teams said that the results were "five sigma," meaning there was just a 0.00006% chance that what the two laboratories found is a mathematical quirk.
"The results are preliminary but the five sigma signal at around 125 GeV we're seeing is dramatic," said Joe Incandela, spokesman for one of the two experiments.
"This is indeed a new particle. We know it must be a boson and it's the heaviest boson ever found. The implications are very significant and it is precisely for this reason that we must be extremely diligent in all of our studies and cross-checks."