Astronomers have used data from NASA's Chandra X-ray Observatory to study the properties of dark matter, the mysterious, invisible substance that makes up a majority of matter in the universe. The study, which involves 13 galaxy clusters, explores the possibility that dark matter may be more "fuzzy" than "cold," perhaps even adding to the complexity surrounding this cosmic conundrum.
For several decades, astronomers have known about dark matter. Although it cannot be observed directly, dark matter does interact via gravity with normal, radiating matter (that is, anything made up of protons, neutrons, and electrons bundled into atoms). Capitalizing on this interaction, astronomers have studied the effects of dark matter using a variety of techniques, including observations of the motion of stars in galaxies, the motion of galaxies in galaxy clusters, and the distribution of X-ray emitting hot gas in galaxy clusters. Dark matter has also left an imprint on the radiation left over from the Big Bang 13.8 billion years ago.
However, astronomers have been struggling for decades to understand the detailed properties of dark matter. In other words, they would like to know how dark matter behaves in all environments, and, ultimately, what it is made of.
The most popular model assumes that dark matter is a particle more massive than a proton that is "cold", meaning that it moves at speeds much smaller than the speed of light. This model has been successful at explaining the structure of the universe on very large scales, much bigger than galaxies, but it has problems with explaining how matter is distributed on the smaller scales of galaxies.
For example, the cold dark matter model predicts that the density of dark matter in the center of galaxies is much higher than in surrounding regions close to the center. Because normal matter is attracted to the dark matter, it also should have a strong peak in density at the center of galaxies. However, astronomers observe that the density of both dark and normal matter in the center of galaxies is much more evenly spread out. Another issue with the cold dark matter model is that it predicts a much higher number of small galaxies orbiting around galaxies like the Milky Way than astronomers actually see.
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