Nowadays people have messed up all the concepts. A layman in science takes the words "power", "energy", "force" to mean the same thing. In the same way, people today take dharma and dharmakaya to mean the same. Both are all pervading, present in all things, etc, etc. Don't interpret dharmakaya literally. It is not the body of dharma, for dharma has no body. It is not the source of dharma either, for dharma has no source. Dharma is the law of nature. Then what is dharmakaya? I'll try to explain using my limited understading.
The Buddhist Trikaya refers to the three bodies that a Buddha has: First, the physical body, visible to naked eyes. Second is the Buddha-body, the never-degenerating-vajra-body of a Buddha that is free of "disease, old-age, decay and death". It is the "immortal body" that is in fact also mentioned in Taoist "internal alchemy". It can manifest in this material realm, but its true form is not visible to any being in the triple world. Only an enlightened being can see it. Pehaps thats why Buddha said: "HE whosees Tathagat(in his true form) sees the Dharma, and vice-versa". You have heard that each Buddha has a Paradise or a 'Pure Land', which results from his enormous merit. That land is diferrent from the heavens within the triple world of suffering, because: beings are deathless, there is no suffering over there, and the paradise is not affected by calamities. The form in which a Buddha manifests in the realm is called the Buddha-body. It is the Main Body of a Buddha, through which the Buddha directly operates. The third body is the dharmakaya, or the "Law Body". A Buddha can, in fact, have more than one Law bodies. I don't know which one, but i think in some Buddhist scripture it is written that Buddha Amitabha has millions of Law Bodies. Law bodies are independent beings that carry the image, nature, wisdom and divine powers of the Buddha-body in them. But they are not completely independent. They are connected (through subtle links) to the Buddha-body, and are willed manifestations of the Buddha. They generally move at Buddha's command, though, if they find some extremely good thing to do, or some urgent matter to handle, they will do it on their own. After Buddha passed away, his Law Bodies are to look after his disciples. Law-bodies are connected to the Main Buddha-body, and are indistinguishable from it. The Main Buddha-body and other Law-Bodies form an inseparable whole. Law-bodies can be treated as extensions of the Buddha-body. The Main Body usually resides in the Paradise, while various tasks, (like looking after disciples) are carried out by Law-bodies. That is why Buddha said that after he passes away, disciples will know him through his Law-body or Dharmakaya.
This is my limited understanding, as I am not a Buddha.