I have an Idea... As someone suggested earlier; magic could be present naturally in the universe. Magic is a source of energy, it could be life, the light, heat... The source of magic radiation could be a star, life or the plane (the current universe) itself.
Let take the star as an example: everyday it produce a large amount of energy. One part is directed toward the world. Naturally, the environment stock that energy. Generally, the capacity is limited and when the level of saturation is attained, the energy flow toward lower energy sites, just like the wind blow from high to low pressure zones. Lifeforms absorbs energy too, probably in greater quantity. So most people have magic energy inside them but have no idea how to use it. When casting a spell, a mage could use his surroundings to fuel his spell. he could go as far as killing other lifeforms if he pushes too hard.
Magic could exist in the form of thermal energy; it mean that casting a spell will make the area colder. The hotter the place, the more powerful the spell can be...
Or it could be more like in the Forgotten realms: magic exist in the world and replenish over time but we are not sure of it's source.
Using magic would then be dangerous to use. The mage have to stock energy inside him , and that alone could hurt him. And using spells that way will also tire the mage.
One could use a catalyst in order to gather magic energy for future uses. It's safer but also limited in capacity.
Different forms of magic could exist. For example, one that is based on the shadow, the moon, or that other star (it's a far and much colder star).