The zero-energy universe hypothesis proposes that the total energy of the universe is zero, and that matter is essentially 'positive energy', and the gravitational field represents 'negative energy', and over the scale of the universe the two cancel each other out.
Hunted through 'A Brief History of Time', which I was sure mentioned this concept, and found this:
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Two pieces of matter which are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together. Thus, in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels out the positive energy represented by the matter. So the total energy of the universe is zero.
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