Controversial and unpopular thought... but maybe, just maybe, relying on something that cannot be seen or detected to fill in the gaps to make models work might mean that there are more fundamental issues with the models themselves?
From the perspective of General Relativity, 'dark energy' is not any kind of defect in the model at all.
Any mathematical model for physics will have a number of free parameters which need to be determined experimentally. GR, astonishingly, really has only one such parameter (the 'standard model' of particle physics has 19). Well, as it's normally written it has more than this: it has three:
Rab - Rgab/2 + Λgab = (8πG/c4)Tab
The constants here are G, Newton's gravitational constant, c, the speed of light, and this Λ (uppercase lambda) thing. R, g and T are not free parameters: R and g measure the curvature of spacetime and T measures the energy-momentum that curvature corresponds to: those quantities are the things the theory relates.
But two of those parameters are in fact not really parameters: they're there simply because people have chosen bad units. Because we didn't know that time should be measured in metres (or distance in seconds, it doesn't matter) we chose the unit of time – seconds – to be an absurdly large number of metres, and so if we want to work in units where time is in seconds and distance in metres we end up with this huge fudge factor, c. Similarly G is another fudge factor (this time a very small number in SI units) which is really because we didn't realise that mass should be measured in units of length, and that kg were absurdly tiny unit of length (the mass of the Sun is about 1.5km for instance). So by fixing the units we use to be sensible (sensible by the standard of someone interested in the underlying geometry, not sensible in any engineering sense!), both of those fudge factors go away, and we end up with just one:
Rab - Rgab/2 + Λgab = 8πTab
So all we have left is this mysterious Λ thing. And you really can't make Λ go away: it's a genuine free parameter of the theory. Mathematically, it's really a constant of integration, so it's a thing which actually needs to be measured. Its name is the cosmological constant.
Well, to cut a long story short for a very long time people just blindly assumed that Λ must be zero, really because Einstein originally assumed it would allow GR to support a static model of the universe which people then thought it was, but it won't do that, so he just said that it should be zero.
But there was never a good reason for that assumption: it's a free parameter of the theory and it therefore needs to be experimentally determined. And, well, we now have been able to begin measuring it, and lo and behold it's not zero: it seems to be small and positive. And what this means that that empty space essentially has some tiny amount of positive energy, and that means that, on very large scales, things get pushed apart from each other. And so we invent this rather silly name for it, 'dark energy', as if it is some mysterious thing in the same way dark matter really is a mysterious secret thing. But it's not: it's just a free parameter in GR which turns out not to be zero.
(Particle physicists will differ about this and claim that Λ must arise from QFT, but I'll takt the GR answer that it's a free parameter against the QFT calculations of what it should be which are out by 10120, thanks. Also 19 free parameters, srsly, what kind of cheap shit theory is that?)
Biting the hand that feeds IT
