Familiar forms of matter and energy comprise just 4% of the universe; the balance is Dark Energy [70%] and Dark Matter [26%]. This is the startling conclusion from two recent major research programmes: satellite measurements of minute spatial variations in the temperature of the source of cosmic background microwave radiation, and studies relating the red-shift in the spectra of supernovae to their distance.
What is Dark Matter, and what is Dark Energy? The short answer is that we don't know.
Dark Matter is the easier concept to consider. Previous analyses of motions of galaxies revealed that the gravitational forces involved were much greater than predicted from the estimated mass interacting in the galaxies. The simplest explanation is that galaxies contain much dark matter that is not detected directly by our instruments, although its effects can be seen and measured. So there is growing evidence that dark matter is an observable reality, with the prospect that its nature will be determined eventually.
Dark Energy is much more problematic.
It is invoked to explain a further important conclusion from these researches. The evidence suggests that the expansion rate of the universe reduced during the first 8 billion years after the big bang, but has been increasing during the subsequent 6 billion years. Dark energy is invoked as a sort of anti-gravity to explain this acceleration.
So dark energy is an unknown entity introduced to make cosmological equations conform to observations. In short, it is a 'fudge factor'.
'Fudge factors' always reduce confidence in the theory behind the equations: when a 'fudge factor' is 70% of the conclusion the theory must be in serious trouble.
The theory in question is general relativity. Einstein himself found it necessary to introduce a 'cosmological constant' to make his equations balance. Later he said this was his biggest mistake, but maybe he was in advance of his time on this issue too.
Relativity works very well on scales up to galactic. It is in the intergalactic, cosmological scale that difficulties arise. The problem may be that relativity has minor inaccuracies that matter only at the largest scales, in the same way as Newtonian gravitational theory works well for most planetary calculations, but proves inadequate at greater distances and greater gravitational fields.
When a theory is in trouble it is a good principle to consider the things that are generally assumed to be beyond question.
One assumption is that the mass of the universe is constant. Yet everywhere we look we see evidence of matter being converted to energy: particles with mass giving rise to mass-less photons of radiant energy. Such processes power the stars and cause stellar explosions. To be sure, the loss of mass is very small in relation to the energy released, but the universe is big, and the time scale enormous.
Nowhere have we seen evidence of the converse: matter being created from energy. To be fair, this would be more difficult to observe; but (naively perhaps) it does appear that the total mass of the universe is reducing, and the total gravity with it. This may be the factor ignored in current theory, which might - just might - make the hypothesis of dark energy unnecessary.
Further, if matter and mass are being destroyed, with no compensatory reverse processes, then the ratio E/M is increasing [where E is the total energy in the universe, and M the total mass]. Now mass and energy are related to the speed of light according to the Einstein equation, e=mc^2 ; or c=√(e/m). Could it be that the balance of energy and mass in the universe determines the speed of light? If so, it means the speed of light is increasing with E/M: the relation of time to the other dimensions of space is changing.
Increasing speed of light gives a new explanation of the red-shift of the spectra of distant radiation sources, and hints that the apparent expansion of the universe may be an optical illusion.
Is this absurd? I lack the skills and knowledge to examine these questions properly. I take comfort in the thought that it was the simpleton who spotted that the emperor was naked.