Actually, "statistical quantum fluctuation" is a possible answer. The energy content of the Universe appears to be near zero, and may be exactly zero ... in which case an arbitrarily large long-lived quantum fluctuation is allowed by the undcertainty principle.
(The potential energy of two masses an infinite distance apart is zero. The potential energy decreases as you bring the objects closer to each other. Therefore gravitational potential energy is negative and cancels the postive energy of mattter. This is actually a hand-waving argument with one serious hole, but the real math gives the same result and is much harder to understand).
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