The reason the Big Bang Theory is nearly proven is because it is well understood that energy is equivalent to mass multiplied by the speed of light squared which was discovered by Albert Einstein through his most famous equation, E=mc2. At very high temperatures, matter becomes energy and an almost infinite amount of energy can be crammed into an almost infinitely tiny space. This energy/matter however is not entirely stable and therefore it must eventually expand outwards. This principal of physics demonstrates that the entire universe could fit into something the size of a marble or less