Young Wizards is a series of books by Diane Duane in which teenage wizards fight against the Lone Power, which seeks to destroy the universe. In the Young Wizards universe, magic exists, and across the wide span of the universe it works in cohabitation with science to make interstellar and intergalactic life possible. Its purpose is to combat the spread and progress of entropy: the long, slow death of the universe, which was put into motion by the Lone Power. Since this is a battle being fought against an entity which really doesn't exist inside time as we know it, it's a very long fight, and one that has the potential to make your head hurt if you think about it too hard.