Scientists say they believe they have captured the elusive Higgs boson - the particle thought to endow matter with mass.
Today's news was broken at a packed seminar at the Geneva headquarters of Cern, the European Organisation for Nuclear Research.
Cern is also the home of the Large Hadron Collider - the 'Big Bang' particle accelerator which recreates conditions a billionth of a second after the birth of the universe.
There is a strong Welsh connection to the groundbreaking scientific project as Dr Lyn Evans from Aberdare was the project leader.
In a telephone call to ITV Wales from Cern, the former Swansea University student spoke of his pride that Wales was involved in the discovery.
Professor John Womersley, chief executive of the Science and Technology Facilities Council, told reporters at a London briefing that they had discovered a particle consistent with the Higgs boson.
Discovery is the important word. That is confirmed. It's a momentous day for science.
Named after Peter Higgs - the Edinburgh University physicist who first proposed its existence in 1964 - the Higgs boson has become one of the most elusive particles in modern science.
Prof Higgs, 83, dreamed up the concept in a moment of inspiration while walking in the Cairngorms.
Two scientific papers followed, the second of which was initially rejected and then finally published in the respected journal Physical Review Letters.
Prof Higgs's groundbreaking proposal was that particles acquire mass by interacting with an all-pervading field spread throughout the universe. The more they interact, the more massive and heavy they become.
A 'boson' particle was needed to carry and transmit the effect of the field - the Higgs boson.
In December last year scientists at the Large Hadron Collider revealed they had caught a first tantalising glimpse of the Higgs.
Since then they have sifted through vast quantities of data from innumerable high energy collisions in an effort to reduce the odds of being wrong.
Proving the existence of the particle has been described as similar to drawing gradually closer to a familiar face seen from afar.
Finding the Higgs plugs a gaping hole in the Standard Model - the theory that describes all the particles, forces and interactions that make up the universe.
Professor Peter Higgs, the retired British physicist from Edinburgh University after whom the particle was named, was in the Geneva audience.
He wiped a tear from his eye as the findings were announced.
I would like to add my congratulations to everyone involved in this achievement. It's really an incredible thing that it's happened in my lifetime.
Speaking at Prime Minister's questions in the Commons, David Cameron hailed Britain's 'contribution to this extraordinary breakthrough'.
It is a very big step forward and we should congratulate everyone involved.