One of the great puzzles of the universe is why it is mostly made of one kind of matter instead of equal amounts of matter and anti-matter.

Matter and anti-matter are counterparts. Bring them together and they annihilate each other in a burst of energy.

It is believed that the cosmos was formed with equal amounts of matter and anti-matter but today the universe is overwhelmingly made of matter. Anti-matter is rare.

The new "particle smasher" at Stanford University has been built to explore the differences between matter and antimatter.

Scientists circulated beams of electrons and their antimatter counterparts in two rings, each more than a mile in circumference.

When they brought the narrow beams of particles together at a single crossing point, they observed the deflection and disruption of one beam by the other - a sign that head-on collisions between the electrons and positrons had occurred.