Evolution of the universe. Computer illustration of the evolution of the universe. Time passes from left to right, with the present at centre left. The universe is thought to have been formed in the Big Bang (far left), believed to have occurred 12-15 billion years ago. The atoms which formed as the universe cooled from this huge explosion are at lower left. These atoms coalesced into stars, gases and galaxies. As the universe ages it expands, and star formation slows down. Existing stars age and die, leaving voids filled with cold gases. This phase, at centre right, may occur 100 billion billion years after the Big Bang.