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Dark Matter Detected?

January 8, 2015

As a child, the only chocolate I knew was milk chocolate, an inexpensive chocolate composition having just 10% chocolate content by US standards. As its name implies, the chocolate is diluted with milk, typically condensed milk. Later in life, I discovered dark chocolate, which has a much higher chocolate content and a much more satisfying taste, at least to me. Although sold as dark chocolate, it's also known as semisweet, or bittersweet, and its composition varies.

Dark chocolate

Dark chocolate.

Women seem to crave chocolate, but solid evidence that chocolate mollifies hormonal imbalance is lacking.

(Modified Wikimedia Commons image by S. Kopp.)

Astronomical observations over the past several decades indicate that we might be living in an analog of a milk chocolate universe in which the visible matter that we hold dear is diluted. Conventional matter constitutes a mere 5% of the composition of the universe, the rest being dark matter (23%) and dark energy (72%). Dark matter, formerly known as the "missing mass" of the universe, shows its presence in the gravitation that binds galaxies together, while dark energy causes the acceleration of universal expansion.

Composition of the Universe

Universe in hiding.

The visible is just a small part of our universe.
Dark energy and dark matter dominate.

(NASA Illustration / WMAP Science Team).[1)]

Not knowing the nature of 80% of the mass of the universe is a big deal, and explaining dark matter has been the principal occupation of many astronomers over many years. I wrote about dark matter in many previous articles (Whither WIMPs, November 22, 2010, A Dearth of Dark Matter, April 23, 2012, Local Dark Matter/A Hundred Years of Cosmic Rays, August 17, 2012, Anapole Dark Matter, June 24, 2013, and LUX Dark Matter Search, November 20, 2013).

Unlike other astronomical objects, we've never observed dark matter directly, since it didn't appear to either emit or absorb electromagnetic radiation. Dark matter was just inferred by its affect on the orbital velocities of stars about our galactic center, among other gravitational effects.

There may now have been a direct detection of a dark matter particle. Two teams of astronomers have reported the existence of a weak
X-ray emission line that's not a known atomic emission line in the Andromeda Galaxy and the Perseus galaxy cluster.[2-6] Although this emission line is at the limits of their instrument's detection, it has the hallmarks of a dark matter decay line. It's stronger towards the center of the objects, and it's absent in a "blank sky" spectrum.[2-6]

Andromeda Galaxy

The Andromeda Galaxy.

This NASA image is my current desktop computer background.

(NASA image)

The 3.52±0.02  keV line was detected by two spacecraft, the XMM-Newton and the Chandra X-ray Observatory. The XMM-Newton team had participants from Leiden University (Leiden, Netherlands), the École Polytechnique Fédérale de Lausanne (Lausanne, Switzerland), and the Bogolyubov Institute of Theoretical Physics (Kiev, Ukraine). The Chandra team had participants from the Harvard-Smithsonian Center for Astrophysics (Cambridge, Massachusetts), the NASA Goddard Space Flight Center (Greenbelt, Maryland) and the University of Maryland (College Park, Maryland) .

Shortly after the Chandra X-ray Observatory and XMM-Newton were launched in 1999, some tests of certain dark matter theories using their X-ray capabilities were proposed. In one of these, keV-mass dark matter particles would reveal themselves through decay into keV X-ray photons. That theory supposes that dark matter particles are so-called "sterile" neutrinos; that is, neutrinos that don't interact via weak interaction, only by gravity.

Such particles can interact with normal neutrinos through a neutrino oscillation that occurs as a consequence of sterile neutrinos' not having a definite mass, so they're a mixture of different neutrino mass states. The sterile neutrino, ν2 will decay into an X-ray photon, γ, and a neutrino of a lighter mass, ν1, producing a line in the X-ray spectrum (see figure).[6]

Feynman diagram of possible dark matter decay

Feynman diagram of possible dark matter decay.

(Adapted from an American Physical Society image using Inkscape.)

One problem with observing this decay is that its lifetime is about 1021 years. However, the Perseus cluster has a dark matter mass of about 1014 solar masses. This translates to more than 1077 dark matter particles of keV mass. Cranking through the numbers gives 1048 decays per second for the Perseus cluster.[6] Still, such radiation should be emitted isotropically, with just a few photons in Earth's direction, so it's only now that an X-ray signature has been detected.

Combined observations lead to an estimate of statistical significance of 4.4-σ; that is, there's 99.999% confidence that this is a real signal.[6] Whether the signal is a dark matter signature is another matter. The XMM-Newton team cautiously writes,
"Although for each object it is hard to exclude that the feature is due to an instrumental effect or an atomic line, it is consistent with the behavior of a dark matter decay line. Future (non-)detections of this line in multiple objects may help to reveal its nature."[2-3]

It's been suggested that the X-ray line could be atomic lines of potassium-18 and chlorine-17, but the Chandra team refutes this idea.[7] The line is observed in Andromeda, which shouldn't have gas hot enough to excite those lines. Furthermore, the line is more intense in objects that should have a greater dark matter content.[6] Knowledge of the universe is gained mostly through advances in instrumentation. The Astro-H X-ray Telescope, planned for launch this year, or early next year, has improved detection capability to confirm this dark matter detection.[6]


  1. WMAP- Content of the Universe, NASA Web Site.
  2. A. Boyarsky, O. Ruchayskiy, D. Iakubovskyi, and J. Franse, "Unidentified Line in X-Ray Spectra of the Andromeda Galaxy and Perseus Galaxy Cluster," Phys. Rev. Lett., vol. 113 (December 15, 2014), Article No. 251301, DOI: http://dx.doi.org/10.1103/PhysRevLett.113.251301. This is an open access paper with a PDF file available here.
  3. Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, and Jeroen Franse, "An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster," arXiv, February 17, 2014.
  4. E. Bulbul, M. Markevitch, A. Foster, R. K. Smith, M. Loewenstein, and S.W. Randall, “Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters,” Astrophys. J., vol. 789, no. 1 (July 1, 2014), Article No. 13, doi:10.1088/0004-637X/789/1/13.
  5. Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, and Scott W. Randall, "Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters," arXiv, June 9, 2014.
  6. Kevork N. Abazajian, "Viewpoint: X-Ray Line May Have Dark Matter Origin," Physics, vol. 7 no. 128 (December 15, 2014), DOI: 10.1103/Physics.7.128.
  7. Esra Bulbul, Maxim Markevitch, Adam R. Foster, Randall K. Smith, Michael Loewenstein, and Scott W. Randall, "Comment on 'Dark matter searches going bananas: the contribution of Potassium (and Chlorine) to the 3.5 keV line'," arXiv, September 15, 2014.

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