Image: Bullet Cluster
A composite image of the Bullet Cluster, a much-studied pair of galaxy clusters that have collided head on. One has passed through the other, like a bullet traveling through an apple, and is thought to show clear signs of dark matter (blue) separated from hot gases (pink).
By Staff Writer
updated 10/8/2007 11:21:47 AM ET 2007-10-08T15:21:47

Dark matter is a mysterious something invoked by scientists to explain mass they know is out there but which can't be seen. The invisible matter, far more prevalent than regular matter, is evident by its gravitational effects on galaxies. And as researchers hunt for the strange stuff, they need not worry about it disappearing any time soon, if that makes any sense.

New calculations show at least 2.1 million-billion years must pass for half of the invisible stuff to decay, if it does at all.

While all this theoretical thinking seems awfully abstract, it represents yet another effort to pin down what the heck dark matter might really be.

Slow decay
Scientists proposed the half-life — about 150,000 times longer than the current age of the universe — after looking at X-rays from the Bullet Cluster, a cosmic collision of galaxies thought to harbor two massive globs of dark matter.

If dark matter can slowly decay, it can also emit radiation, albeit at nearly undetectable levels. The proposed ultra-wimpy signal might help explain why it's practically invisible to our scientific instruments.

"We don't know what dark matter is, but we do know it's made of some kind of particle," said Signe Riemer-Sorensen, an astrophysicist at the University of Copenhagen in Denmark. "One theory says these particles are axions, which are very massive and can decay."

Axion decay, she explained, is similar to radioactive decay of unstable elements, such as uranium-238. "But instead of different atoms, you get two photons we might detect as X-rays," she said.

Clear evidence

Chandra X-ray telescope
The Bullet Cluster in X-ray. The green contours surround large masses; to the right of visible matter (colored) is an large mass thought to indicate an enormous collection of dark matter. Very few X-rays are emitted from the region, however, leading astrophysicists to think dark matter decays very slowly
Riemer-Sorensen and her team searched for the X-ray hallmark of decaying dark matter in the Bullet Cluster. The formation is one of only a handful thought to show clear evidence of dark matter, which makes up about 85 percent of all matter in the cosmos.

The astrophysicists compared X-ray emission from one of the cluster's dark matter blobs to a desolate region nearby, but hardly any difference showed up.

"We didn't see anything," she said, explaining that the dark matter blob had about the same amount of X-rays that the "control" region without any dark matter did. "This leads us to think dark matter must take a very long time to decay."

While Riemer-Sorensen said her team can't be certain about their conclusion, she explained that Chandra — the space telescope used to measure the Bullet Cluster's X-rays — may not be sensitive enough to "see" the decay of axions.

"There could be a weak X-ray emission from axion decay we aren't able to detect yet," she said, noting that more sensitive X-ray observatories are necessary to do so. "It's impossible to say it's not there until we have better instruments."

The team of astrophysicists detailed their findings in a recent issue of the journal Physical Review Letters.

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