Zero Point energy, or the energy of empty Space Time, gives physicists fits. One the one hand, observation shows a tiny, tiny amount of energy. (This is the probable source of the "dark energy" that pushes distant galaxies apart.) On the other hand, the calculations show an enormous amount of energy. How enormous? 120 orders of magnitude more than the observations show. That is a lot!
This discrepancy is easily solved. The calculation show the total amount of energy in empty space. This is the amount of available energy in space-time, from which all other fields draw energy. (Gradients in the space-time energy field have an effect that we call gravity. You have to understand gravity as a push from empty space, not a pull by matter.) This energy source must, of necessity, be enormous. It is also relatively equal at all places.
Relatively equal - there's the source of the problem. The uncertainty principle tells us that nothing can be completely equal all the time. And thus the "surface" of space-time is awash with tiny differences. These aggregate into waves with real, albeit tiny amounts of energy. How much energy? About 120 orders of magnitude less than the total amount held in space-time.
Traversing these frothy waves is what reduces the energy of photons traveling intergalactic distances. The aggregate addition to the energy of space-time over remarkably long distances is enough to accelerate galaxies away from each other.
Once you understand the true nature of gravity and space-time, everything else makes perfect sense. There are no contradictions in nature, merely in our flawed understanding.