by Amy Joy Hess

They are still hunting for it, that mysterious invisible particle we’re told makes up 85% of the matter in the Universe. Ever since Fritz Zwicky suggested dunkle Materie to explain why the Milky Way and other spiral galaxies aren’t flying apart, physicists have been on the hunt for dark matter. Decades and billions of dollars later, they still haven’t found undisputed evidence for the elusive stuff, and yet as NASA’s Chandra X-ray observatory recently enjoyed its 15th anniversary, dim possibilities offer hope to those scientists desperate to find the material that holds the Universe together.

Young scientists must not fear; there are still plenty of puzzles left to solve. Based on spectral line data, it appears that the outer rims of spiral galaxies are moving at the same rate as the insides of the galaxies — and that doesn’t make any sense. The galaxies should fly apart from spinning that fast. Starting with Zwicky in the 1930s, physicists have hypothesized the existence of an invisible material that provides the gravitational pull to hold the galaxies together. They imagine a fabric of unseen particles working to keep, not just galaxies, but clusters of galaxies from shooting apart as they swirl in a massive cosmic dance.

But, this material doesn’t emit or absorb light. We can’t see it. Groups of physicists have had huge accelerators built to smash particles into it with the hopes of getting dark matter to show its face for a fraction of a second — with no real success.

Pijushpani Bhattacharjee at Saha Institute of Nuclear Physics in West Bengal, India has spent 20 years working on high energy cosmic rays deep in the Universe. In 2008, he joined the Picasso experiment in Canada to add his efforts to the hunt for dark matter. The Picasso scientists developed a method that uses superheated liquid to detect the invisible substance. Theoretically, dark matter particles would create sound waves when they hit droplets of specially engineered liquid, but while the scientists were expecting to get at least a few “dings” a year, they’ve detected nothing notable in five years of fine-tuning.

Cosmologists have a variety of reasons for embracing the idea of dark matter, and they are confident that dark matter comes in the form of a particle, a weakly interactive massive particle (WIMP) that creates gravitational effects but otherwise ignores normal visible particles. The trick is to get it to get some WIMPs to show themselves by hitting visible matter into them and making them say, “Ow!”

Rick Gaitskell of Brown University has been hunting for dark matter for some 25 years and heads the team that turned on the Large Underground Xenon (LUX) experiment in South Dakota. A mile underground in the Homestake Gold Mine, the LUX particle accelerator shoots xenon particles past ultra-sensitive detectors. If the xenon particles smack into one of these WIMPs, it should give off a little flash of electricity that the detectors can catch and record.

So far, though, the LUX hasn’t found anything. Gaitskell told Popular Science last fall, “Every experiment has reported essentially negative results. No one even knows for sure if the d- stuff really exists.” If dark matter really does make up five-sixths of the matter in the Universe, it certainly does an excellent job of hiding itself.

Sterile Neutrinos

There’s still hope, though. After all, there is a true explanation for why spiral galaxies haven’t flung apart. While hunting through X-ray emission lines from 73 galaxy clusters, Harvard Smithsonian Center for Astrophysics (CfA) astronomers and their colleagues recently found a faint spectral line that doesn’t apply to any known atomic transitions. They have been cautious, wanting to make sure the line was not just caused by their instruments or other factors. Still, the line was detected at both the Chandra observatory in Huntsville, Alabama and Europe’s XMM-Newton observatory. Scientists in the Netherlands, using the European Photon Imaging Camera (EPIC) of XMM-Newton also found a dim X-ray line at 3.5 keV, which gives independent corroborating evidence.

Both groups of scientists suggest that the line could come from the decay of sterile neutrinos, a top candidate for dark matter. More experimentation must be done, though, because there are other explanations as well. The more objects they analyze, consistently offering a line that fits into the expected dark matter spectral range, the better. A. Boyarsky et al. noted in the Netherlands paper:

The X-ray spectra of astrophysical objects are crowded with weak atomic and instrumental lines, not all of which may be known. Therefore, even if the exposure of available observations continues to increase, it is hard to exclude an astrophysical or instrumental origin of any weak line in the spectrum of individual object. However, if the same feature is present in the spectra of a number of different objects, and its surface brightness and relative normalization between objects is consistent with the expected behavior of the DM signal, this can provide much more convincing evidence about its nature.

The Dark Herring

Of course, dark matter may not exist after all. In his own PowerPoint slides on dark matter posted on the Brown University website, Gaitskell tells his students, “It has been a Problem in Cosmology that astrophysical assumptions often need to be made to interpret data/extra parameters.” It’s true. Scientists create models they use to interpret the information that space gives them. The models are based on certain assumptions, and if those assumptions are incorrect, the data gets interpreted wrongly. There may be entirely different explanations for the survival of spiral galaxies or gravitational lensing, explanations that don’t require invisible matter we can’t see or detect.

The nature of the Universe is an involved mystery, a deep subject that requires a great deal more study. Yet, the hunt for dark matter highlights the importance of examining one’s assumptions in the pursuit of scientific truth. Astrophysicists need to be able to interpret data, but a great deal of time and money can be spent to prove incorrect interpretations when the underlying assumptions are misdirected.

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