Even though it's suspected of making up about 85 percent of the matter in the observable universe, dark matter is a mysterious beast. It's effectively invisible and can only be detected through its interactions with ordinary matter, but that hasn't stopped scientists from trying to understand just what it is and how it behaves. Now, using data gathered from NASA's Chandra X-ray Observatory, astronomers have found that the best way to explain how matter clumps together in galaxies is with a model of dark matter that's both fuzzy and excited.

Dark matter gets its name from the fact that it doesn't interact with light at all, which renders it invisible to us. In fact, we only know (or at least, strongly suspect) that it exists because its gravity affects the visible parts of the cosmos, and it conveniently plugs a few other holes in our understanding of the universe. Past research has suggested that it forms "hairs" around planets, connects galaxy clusters in filaments, and could play a key role in Earth's mass extinction cycle.

The most commonly accepted theory says that dark matter is "cold", which means that its particles move much slower than the speed of light. This theory is useful because it helps explain how the universe became so "lumpy." Shortly after the Big Bang, the universe is thought to have been relatively smooth, with matter distributed fairly evenly, but today it tends to clump together into galaxies and clusters of galaxies. If dark matter is cold, then as the universe expanded, pockets of it slowed down and eventually recollapsed, creating more dense sections. Ordinary (or baryonic) matter is attracted to dark matter so it will tend to gather in these areas as well, creating galaxies as we know them.

But handy as it is for explaining the structure of the universe on an intergalactic scale, the cold dark matter theory falls apart when it's applied to how matter is distributed inside a given galaxy. According to the theory, both dark and regular matter should be most dense right in the center of a galaxy, but observations show that isn't the case. Instead, it tends to spread out more evenly.

Fuzzy dark matter, on the other hand, helps solve that problem. For this work, the researchers assumed that dark matter is made up of extremely light particles, with a mass about ten thousand trillion trillion times smaller than that of an electron. If this is the case, the particle's wavelength of light would be about 3,000 light-years long, meaning if you could see these particles, they would appear very fuzzy – hence the name.

The theory of fuzzy dark matter itself comes in two flavors: in the more simple version, all particles have the lowest possible energy, but a more complex version says that the particles can have different amounts of energy, called "excited states."

For this study, the astronomers applied both models to 13 galaxy clusters, to test how well the theories explained what was observed. Based on data gathered by Chandra, the team estimated how much dark matter would be present in each cluster, and how it's spread out from the center. Based on that data, the researchers concluded that the more simple model doesn't work, but the excited states model does agree with observations, in some cases even better than the standard cold dark matter idea.

While the researchers are encouraged by their findings, they point out that more work needs to be done to properly test the model. In particular, they say that the excited states should produce ripples that can be detected in the density of normal matter, and spotting this effect would help confirm the theory.

The research was published in the Monthly Notices of the Royal Astronomical Society.

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