New cosmic theory unites dark forces

Although the fluid is all around us, Zhao found that it does not affect the motion of Earth or the other planets, which is "reassuring," he said, because data shows that our solar system obeys traditional gravity to very high accuracy.

But the fluid does affect the speed at which galaxies can rotate. Some 75 years ago, astronomers noticed that galaxies were turning faster than would be expected from the amount of normal light-emitting matter they contained. The answer seemed to require some form of unseen dark matter.

However, Zhao has shown that his fluid can keep galaxies from flying apart just as well as dark matter can.

Zhao has also tested his model against the bullet cluster of galaxies, where a massive collision appears to have stripped hot gas from its dark matter envelope. This "naked" dark matter was seen as iron-clad proof for traditional dark matter theories, but Zhao claims that his fluid can reproduce the same effect.

Christian Boehmer from University College London thinks it "compelling" that Zhao's model can reproduce so much galaxy data.

Word search
If the dark fluid is mimicking dark matter, then scientists are searching in vain for the elusive dark matter particle, often called a WIMP (for weakly interacting massive particle).

Currently, several experiments are trying to detect a rare collision of a WIMP on Earth or observe gamma rays from distant WIMP self-annihilations. 

"Direct detections will be more difficult," Zhao said. WIMPs may still exist, but there won't be as many of them as predicted.

Without WIMPs to worry about, the dark fluid could make scientists' jobs easier.

But not many cosmologists are ready to abandon dark matter just yet. The dark fluid idea is still fairly new, so some issues have yet to be worked out, whereas dark matter is a fairly mature theory.

"The current [dark matter] model provides the best fit to the data and is therefore the best model at hand," Boehmer said.

However, Boehmer agrees that having two unknowns — dark matter and dark energy — make up 95 percent of the universe is a bit embarrassing for cosmology.

"Frankly speaking, these are just fancy words we use to name something we do not understand," he said.

If a simpler model (with a single word) can explain all the data, then cosmologists will gladly accept it, Boehmer said.

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