Discovery by Newton
Belgiumgium was first found in a Belgian chocolate by Isaac Newton. Mr. Newton was deathly fond of chocolates; he ate everything with them. He gained some fame for winning the "Most Disgusting Dinner Menu" Award in 1663 by way of his short-lived restaurant's ("Newton's") dinner menu which among other things featured a full-course meal made entirely of chocolate (chocolate salad, beef with hot whipped chocolate, chocolate balls with chocolate sauce, etc.).
While searching for tasty confectionaries with which he would anoint his menu of decadence, he decided to try the Belgian variety of chocolate (which was rumored to make its consumers lighter, thus courting popularity among fat chicks). Newton was feeling somewhat under the weather after trying one of Count Chocula's Kit Kat Bars; he had developed a terrible stomach ache, probably as a result of the Kit Kat Bar aggrevating his already gluttonous tendancies toward chocolate.
For the first time in his life (barring that incident at Shennanigan's), Newton passed up chocolate and attempted instead to analyze the chocloate using scientific methods. After much rigorous testing and salivation, Newton discovered that Belgian chocolate contained a previously undiscovered element which he named Belgiumgium, the secret 114th element mentioned in the Book of Revelation.
Belgiumgium-Billiards Theory of Two Cows
Belgiumgium has minus mass which might be related to tachyons or String Theory. Newton, though, didn't discover either through his studies of Beligiumgium, and since Newton discovered practically everything, he would have found whether the three were related.
Since Belgiumgium has minus mass, it is repelled by the earth's gravity and retreats higher and higher into the earth's atmosphere. This is why naturally-occuring Belgiumgium is nearly impossible to find, given that its repulsion to plus mass means that little of it, if any, would be involved in the creation of planets. The question Newton then raised was: if Belgiumgium is so hard to find in nature, why is it found with such frequency in Belgium?
His resulting theory (known as Newton's Belgiumgium-Billiards Theory of Two Cows) suggested that, somewhere in the universe, there was a planetoid of Belgiumgium, and that this planetoid came in violent contact with a much larger planetoid composed of plus mass. The resulting impact repelled the Belgiumgium planetoid at a great speed causing it to collide with an infant Earth in its molten stage; the viscosity of a molten Earth allowed the Belgiumgium to penetrate its surface until it was wholly absorbed, the Earth cooling afterwards.
As Belgiumgium has minus mass, it gradually moved through the Earth's layers until it began to rise up from underneath the area it originally struck - the general area of the nation of Belgium. From here, various plants (particularly grass) absorbed Belgiumgium. The Belgiumgium-enriched grass was eaten by various herbivorous fauna, thus beginning the Belgiumgium cycle, in which animals consume Belgiumgium and excrete it or die, either way enriching the earth's soil anew with Belgiumgium. Over the years this caused a feedback effect with the grass of Belgium (and its consumers, and consumers of its consumers, etc.) becoming hyper-enriched with Belgiumgium.
As mammals (particularly cows) secrete Belgiumgium through their milk, Belgian chocolate is enriched with Belgiumium. The theory is called the Belgiumgium-Billiards Theory of Two Cows because, well, if you don't have two cows (of differing sexes), then you can't make chocolate for very long, can you?