PARIS, Oct 7, 2008 (AFP) - Humans like the comfort of symmetry -- the identical image in the mirror, the matching wings of the baroque mansion, the equal numbers in opposing football teams.

So it comes as a bit of a shocker when physicists say the Universe is built on broken symmetry.

Creation was not a soothing, balanced event, they say. It was, essentially, a lopsided affair.

Had things been symmetrical in the Big Bang 13.7 billion years ago, equal amounts of matter and antimatter should have been formed, rather like the hole you dig in the ground is equal to the mound of earth that comes from the hole.

The problem is that matter and antimatter are deadly rivals.

When a matter particle collides with its opposite-charged foe, the two annihilate each other in a puff of energy. To use the image of the hole, the mound will fill the hole you have dug.

But something happened in the seething soup of primal particles in the instant after the Bang. Matter gained the upper hand over antimatter.

Thanks to this excess of matter, we have the galaxies, the stars, Earth and all the life on it. Without this mysterious victory, we wouldn't be here.

How to explain the enigma lies at the heart of work that earned two Japanese and an American the 2008 Nobel Physics Prize on Tuesday.

Their achievement, in exploring the violation of symmetry, strengthened and widened the conceptual model of fundamental particles and forces, the Nobel Prize committee said.

'Every particle of matter has an opposite number, the antimatter particle,' Etienne Auge, deputy director of the National Institute of Nuclear Physics and Particle Physics (IN2P3), told AFP.

'What is strange, though, is that we are living in a world that consists almost entirely of matter.'

Yoichiro Nambu of the United States earned half of the award for theories developed in the 1960s about 'spontaneous symmetry breaking.'

This underpins the notion that shortly as the Universe started to cool after the Big Bang, a single superforce ripped apart and formed three of the four known forces of nature today.

These are the strong force, the weak force and the electromagnetic force, which act, through messenger particles, on the bestiary of indivisible particles that make up matter.

The other two laureates, Makoto Kobayashi and Toshihide Maskawa of Japan, showed that in certain conditions, antimatter does not obey the same rules as matter.

Spotting the anomaly 'is a bit like holding up a book and looking in a mirror, and then realising that instead of seeing reverse writing, you see proper writing,' said Philip Diamond of Britain's Institute of Physics.

This symmetry break could only be explained by the presence of three families of particles known as quarks, Kobayashi and Maskawa suggested. Nearly three years later, their hunch was confirmed in experiments.

Two big things are still missing from the Standard Model, the conceptual vehicle of particle physics today.

One is an explanation as to how particles acquire mass, and the other is an explanation for the force of gravity.

The leading contender for mass is the Higgs Boson, proposed as a ubiquitous, syrupy field that interacts with other particles.

The 'Higgs' is famously being hunted at the Large Hadron Collider (LHC), the massive particle smasher that was unveiled in Geneva last month.

As for gravity, one idea is that there is a particle called a graviton which conveys the force.

What causes gravity 'poses a colossal challenge for physicists today,' the Nobel committee said on Tuesday.