Dual-Universe Model to Explain Dark Energy Taking a page from Brane-world scenarios, it's postulated that, not one, but two, Universes are created at the time of the Big Bang; that are perfect mirror reflections of one another. One, ours, is composed entirely of positive matter/energy, while its exact mirror twin is composed entirely of negative matter/energy. Due to identical initial conditions both contain exactly the same amount of matter/energy. The result is that in all time periods of their evolution the net energy of these mirror Cosmos's is exactly zero, eliminating all ambiguity about energy conservation in this duplex set of Universes. In compliance with exact symmetry, the proper setting of a negative matter/energy Universe is an inverted 4D coordinate system - negative length, height, and width, and reversal of the time coordinate. Consequently, there is no net space-time at any point in the evolution of these back-to-back Universes. To put it simply - from nothing comes nothing. Due to perfect symmetry all the laws of Physics would be identical in both Universes, from the perspective of sentient beings in each Universe, composed of the matter of that Universe. The definition of positive or negative vacuum states is arbitrary, as a sentient being living in either universe has every right to consider his universe to possess a positive vacuum state - an echo of the SR postulate that "all coordinate systems are created equal". Since Standard Model particles are open-ended strings, they would be stuck on the brane walls in either Universe, unable to escape. This would prevent bizarre scenarios where a combined system of equal size clumps of negative and positive matter would accelerate indefinitely in one direction, though not violating energy conservation as the net mass of the system would be zero. But gravitons in either Universe, being closed-loop strings, would be free to travel to the other Universe, where they would have the opposite-sign energy-state to the gravitons in that Universe. In our Universe gravity leads to acceleration (attraction) of celestial bodies towards each other, whether looked at from the viewpoint of Newtonian gravity, General Relativity, or Quantum Gravity - the last entailing the theorized existence of spin-2 gravitons. The accelerated motion of clumps of matter towards one another, in our Universe, from the quantum viewpoint, is thus mediated by the exchange of positive-energy, virtual gravitons between the two matter clumps. From this train of logic it follows that negative-energy gravitons, (whether real or virtual), should mediate a repulsive force between ponderable masses in our Universe. This, ultimately, should lead to accelerated motion of our galaxy clusters, and larger structures, away from one other. Therefore it is speculated that the inflationary expansion of our Universe is driven by a steady inflow of negative-energy gravitons across the brane-boundary from our Universe's mirror negative-matter/energy twin. This scenario would be replicated inside our twin Universe, where our positive-energy gravitons appear as negative-energy gravitons in that Universe. The presence of negative-energy gravitons in our Universe would produce a negative curvature of our metric, in contrast to the positive metric curvature produced by the positive-energy gravitons exchanged between masses in our Universe. This is the GR way of looking at its repulsive effect. Since Brane-world models expect that gravitons from another Brane can diffuse across any point of the Brane wall into our Universe, in principle, this would lead to a uniformly distributed 'gas' of negative-energy gravitons throughout our Universe. In practice, one would expect that these negative-energy gravitons, from the mirror Universe, would preferentially be emanating from those regions of the negative-matter/energy Universe, where matter is clumped. The result is that if both our Universe, and its negative-energy twin, were superimposed on one another, one should see the voids in our universe occupying the locations of the galactic web structure filaments of our twin Universe, and visa-versa. The recently discovered alignment of galactic discs along these filaments could be a consequence of the negative-energy graviton 'pressure' exerted from the voids on all sides around our galactic filaments. Additionally, if the negative space-time curvature induced by the presence of negative-energy gravitons happened to cumulatively match the positive space-time curvature created by all our positive-energy matter/fields, our Universe would be spatially flat on very large scales as is actually observed. And, finally, if vacuum fluctuations in our Universe included equal quantities of negative-energy Standard Model particles, coming from our mirror-Universe, it would exactly cancel the positive-energy contributions from Standard Model particles from our Universe. This would resolve the 120 orders of magnitude discrepancy between the quantum calculation of the vacuum-energy, versus its actual observed value. Home Page