The New Scientist has a series of articles exploring the questions and quandaries facing physics. One of the articles deals with dark energy:
... The first emissary of darkness put a foot in the door as long ago as the 1930s, but was only fully seen in the late 1970s when astronomers found that galaxies are spinning too fast. The gravity of the visible matter would be too weak to hold these galaxies together according to general relativity, or indeed plain old Newtonian physics. Astronomers concluded that there must be a lot of invisible matter to provide extra gravitational glue.
The existence of dark matter is backed up by other lines of evidence, such as how groups of galaxies move, and the way they bend light on its way to us. It is also needed to pull things together to begin galaxy-building in the first place. Overall, there seems to be about five times as much dark matter as visible gas and stars.
Dark matter's identity is unknown. It seems to be something beyond the standard model of particle physics, and despite our best efforts we have yet to see or create a dark matter particle on Earth (see "Trouble with physics: Smashing into a dead end"). But it changed cosmology's standard model only slightly: its gravitational effect in general relativity is identical to that of ordinary matter, and even such an abundance of gravitating stuff is too little to halt the universe's expansion.
The second form of darkness required a more profound change. In the 1990s, astronomers traced the expansion of the universe more precisely than ever before, using measurements of explosions called type 1a supernovae. They showed that the cosmic expansion is accelerating. It seems some repulsive force, acting throughout the universe, is now comprehensively trouncing matter's attractive gravity.
This could be Einstein's cosmological constant resurrected, an energy in the vacuum that generates a repulsive force, although particle physics struggles to explain why space should have the rather small implied energy density. So imaginative theorists have devised other ideas, including energy fields created by as-yet-unseen particles, and forces from beyond the visible universe or emanating from other dimensions.
Whatever it might be, dark energy seems real enough. The cosmic microwave background radiation, released when the first atoms formed just 370,000 years after the big bang, bears a faint pattern of hotter and cooler spots that reveals where the young cosmos was a little more or less dense. The typical spot sizes can be used to work out to what extent space as a whole is warped by the matter and motions within it. It appears to be almost exactly flat, meaning all these bending influences must cancel out. This, again, requires some extra, repulsive energy to balance the bending due to expansion and the gravity of matter. A similar story is told by the pattern of galaxies in space.
All of this leaves us with a precise recipe for the universe. The average density of ordinary matter in space is 0.426 yoctograms per cubic metre (a yoctogram is 10-24 grams, and 0.426 of one equates to about 250 protons), making up 4.5 per cent of the total energy density of the universe. Dark matter makes up 22.5 per cent, and dark energy 73 per cent (see diagram). Our model of a big-bang universe based on general relativity fits our observations very nicely – as long as we are happy to make 95.5 per cent of it up.
Arguably, we must invent even more than that. To explain why the universe looks so extraordinarily uniform in all directions, today's consensus cosmology contains a third exotic element. When the universe was just 10-36seconds old, an overwhelming force took over. Called the inflaton field, it was repulsive like dark energy, but far more powerful, causing the universe to expand explosively by a factor of more than 1025, flattening space and smoothing out any gross irregularities.
When this period of inflation ended, the inflaton field transformed into matter and radiation. Quantum fluctuations in the field became slight variations in density, which eventually became the spots in the cosmic microwave background, and today's galaxies. Again, this fantastic story seems to fit the observational facts. And again it comes with conceptual baggage. Inflation is no trouble for general relativity – mathematically it just requires an add-on term identical to the cosmological constant. But at one time this inflaton field must have made up 100 per cent of the contents of the universe, and its origin poses as much of a puzzle as either dark matter or dark energy. What's more, once inflation has started it proves tricky to stop: it goes on to create a further legion of universes divorced from our own. For some cosmologists, the apparent prediction of this multiverse is an urgent reason to revisit the underlying assumptions of our standard cosmology (see "Trouble with physics: Time to rethink cosmic inflation?").