This age-old question really has a simple answer. Attempts to answer it, however, and attempts to get around implications of the simple answer are often quite convoluted.

According to the mainstream Big Bang idea, our universe abruptly sprang into existence some 15 billion years ago, sending matter careening through space. After millions of years it had cooled enough to coalesce into billions of rotating proto galactic-discs, and much later into stars.