Let's assume that dark energy is uniformly distributed through space (which is one theory) and that dark energy is quantized (as everything else seems to be). We can visualize that dark energy consists of packets of equal size and distance between the packets, so there would be three-dimensional grid of dark energy packets spread through universe. Let's imagine an observer at rest relative to grid and another observer moving. If the observers could measure the distance between packets, the moving observer would measure smaller distance (at the direction of motion) due to length contraction. By comparing these, they would know that one observer is moving relative to universe and other is not, so the question of "who is moving" wouldn't be totally relative anymore?