Dharmadhatu can be translated as “realm of Dharma.” Dharma, as a Vedic concept, influenced the development of the Dharmadhatu idea. In early Buddhist thought, Dharmadhatu was taken to embody the way of things, of the forces compelling the universe. Dharma, as the way, had its own form. The Dharmadhatu was this form.
In Mahayana thought, the concept evolved to mean the true nature of all things. This true nature of things is not a characteristic of matter. The nature of things is an active force, imbuing and penetrating all phenomena. Just as astrophysicists would have no answer for what is outside of the universe, a Buddhist would have no concept of anything outside of Dharmadhatu. The origin and substance of the universe are not primary concerns to most Buddhists. Understanding the universe’s “purpose” and its physical constitution are considered, in large part, to be unnecessary to the task of being freed from suffering.
The Dharmadhatu might seem like the Buddhist answer to “Why are we here?”, but the concept does not have a teleological aspect. The concept of self-creation, through Dependent Origination, is the “why.” The Dharmadhatu is the totality of everything that has been, is being, and will be dependently originated.
The Dharmadhatu concept is heavily featured in the Flower Ornament Scripture, in which reality is described from the perspective of s perfect being who sees the nature of reality as the Dharmadhatu. In the sutra and further developed in the Hua-Yan school, the dharma-body of Vairochana is the boundless Dharmadhatu.