You see, they are trying to account for acceleration, and basic mechanics teaches us that energy doesn't cause acceleration. There are only two components in an equation for acceleration: mass and force. The mass is accounted for; it is the mass of all the objects in the universe. So what we really need to account for is the missing force, not missing energy.
Luckily, the force we are looking for is already accounted for in the equations for the theory of general relativity, it was just misleadingly labelled by Einstein as the "cosmological constant". However, if you look at the equation for this "constant", you will see that it is not a constant at all, but the equation for a force. This is a fifth fundamental force of nature, hiding in plain sight for over a century, but since scientists have been myopically focused on unifying the four fundamental forces we had already discovered, they have no interest in admitting yet another fundamental force.
One interesting fact about this fifth force, though, is that unlike electromagnetism or gravity, the magnitude of this force increases with distance! That property alone is enough to demonstrate that it is truly a separate force, distinct from the other forces we have already studied.
Increasing with distance... Maybe the frame of reference is not the micro but the macro. That the cause and effect is flipped — and the expanding universe is producing the force and not the other way around?