The probably Galaxy formed from a huge, slowly spinning cloud of gas that collapsed under its own gravity. During the collapse, fragments of this cloud separated and then collapsed forming stars and globular clusters. As the remainder of the giant cloud contracted further, it started flattening up into a disc, because of rotation. This final contraction lasted billions of years, during which stars started to form also in the disc. This scenario provides a natural explanation of why there is a significant age difference (about 3 billion years) between the galactic disc and the galactic halo.