Magic is a force of nature - just like gravity. It is always there, effecting the world, and unless you understand what magic is, you will never notice it.
Before Isaac Newton created his laws of physics, no one had ever thought to qualify inertia, or even glimpsed the true potential of the world around them.
The ancient religions of the world once incorporated magic in their rituals, using the miracles to give life and substance to their gods. New religions rise, and the old ones are forgotten, and slowly the knowledge of magic was lost with them. Once a Druid could fix a famine with their arts, but under persecution and time they all died out.
And, after time immemorial has past, the little humans knew of magic has dwindled away. The age of reason and Science came upon the world, and all superstitions were belittled and dismissed. Magic was finally entirely forgotten.
However, all things end with time, and Magic returned to the awareness of the world with a vengeance. It started with a person named Serena Adams. It was caused by an impossible event.
The theory she proved broke the world entire, and made it anew.