Dark energy—the mysterious and poorly understood force that scientists have proposed is somehow causing the universe to expand at an accelerating rate—is almost definitely real, according to a new study, which puts the likelihood of its existence at 99.996%.

In the mid 1990s, two teams of scientists proposed the existence of dark energy when they observed, while examining distant exploding stars called supernovae, that some of them were less bright than expected. Because the light observed from these distant stars was emitted far in the past, the brightness can be used to examine the rate at which the universe has expanded over time. Because of this discrepancy in brightness, the scientists concluded that the universe’s expansion was accelerating, and they named the repulsive force responsible “dark energy,” a discovery for which both team leaders won the 2011 Nobel Prize in Physics.

The claim that dark energy exists was bolstered by what’s called the Integrated Sachs Wolfe effect, named after Rainer Sachs and Arthur Wolfe. The Sachs Wolfe effect states that light from the cosmic microwave background—radiation present throughout the universe that was created by the Big Bang—would become more blue in color as it passed through the gravitational fields of lumps of matter.

This effect was later used as a test for dark energy when scientists realized that they could compare that radiation’s temperature throughout our local universe with maps of where galaxies are located. If dark energy existed, their calculations showed, the maps should correspond, with the radiation’s temperature being dependent on the location of galaxies. That is exactly what they found.


But the effect was so small that many scientists refused to accept the results, instead claiming the finding could be caused by contamination from sources like space dust.

The new study, carried out by researchers at the University of Portsmouth and LMU University Munich, sought to answer these critics by improving the maps used to compare the temperatures with the locations of galaxies by using newer, more accurate datasets. They also conducted new analyses aimed at ruling out contamination in the data. The results, published in the scientific journal Monthly Notices of the Royal Astronomical Society this week, reinforce the original findings, finding a 99.996% likelihood that dark energy exists.

Nevertheless, dark energy, like dark matter, remains that most enigmatic of physical entities: Something we can detect but not understand. We know it’s there, but we don’t know what it is. That’s enough to keep physicists up at night.

A pre-release version of the paper can be viewed here.


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