The Cylons were created by man to make life better throughout the 12 Colonies. They evolved. They rebelled. After a brutal war that lasted 12 years, both sides signed an armistice that ended hostilities. The Cylons left for another world.
No one saw a Cylon for 40 years. And then, one day, the Cylons returned. In a matter of hours, the 12 Colonies were completely destroyed.