Well, if we're going approach it from an empirical perspective, a miracle is an event the cause of which is impossible to know. Which I think the mystery of creation is. Simply being unknowable puts it outside the scope of empiric knowledge and into philosophy/mysticism.
From a metaphysical perspective, there is no reason for anything to be rather than not to be. "Nothingness" is a false concept (as explained by parmenides), but that is an empirical reservation at its core: non-being is impossible simply because we happen to live in a reality that happened to Be rather than not be. WHY existence is the default rather than non-existence, I think we'll never get to know.