The Cross-Multiverse Theory is a common belief of how and why magic exists in Nova. It goes as follows.
At the dawn of time, space, and everything, there have been infinite universes. Each universe has its place in a completely flawless array -- except for two of them. The two misfit universes, one being Nova, and the other Supernova, are miraculous mistakes that should not have been. Supernova is parallel to Nova. They are essentially the same, besides one small difference. Supernova contains everything Nova has -- except for the ability to contain living creatures. It does, however contain a strange gas that Nova lacks, that is believed to be the fumes of magic itself. Due to, perhaps, a simple coincidence, the two universes accidentally collided at some point, even though they were completely static and virtually immovable. This caused several dangerous ripples in reality, time, and space itself. Chaos churned left and right in all of the universes that ever existed and ever will exist, until the time came that Nova and Supernova formed some sort of a bond. Both universes, stuck to each other, were the only imperfection in the endless infinity of a perfect array of universes.
And with the bond of the universes, Nova became dynamic, and life began. The eternal seal between the two universes allowed some power to pass into Nova. And with the dawn of humanity, some have been able to borrow and harness the strange power from Supernova.