Everything we know about dark matter is related to its effect on normal matter, and although it explains the universe very well there are still many doubts about its true nature (and whether it even exists) because it cannot be observed directly.
One of the most popular models suggests that particles of dark matter are "cold", meaning the particles are thousands of times bigger than electrons, moving quite slowly compared to the speed of light. This cold dark matter model matches observations made about the universe but there's still plenty about dark matter that can’t be explained as easily. Now, researchers have suggested that instead, dark matter might be "fuzzy".
In a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society, physicists from the University of California and the National Polytechnic Institute, Mexico City suggest a diametrically opposed approach. In their model, the dark matter particles are about ten trillion times smaller than an electron, and this has some curious quantum mechanic consequences.
A principle of quantum mechanics is that the fundamental building blocks of matter are both particles and waves, although we tend to describe them as just particles for convenience in approximation. So, these tiny dark matter particles are a just a meager fraction of an electron, but their wavelength is 3,000 light-years across.
Having such a long wavelength (about 3 percent of the size of the Milky Way) creates a constant interference between the dark matter particles, a bit like the surface of a pond in a thunderstorm. On these large scales, the quantum mechanical properties have an impact on galactic rather than atomic scales.
The cold dark matter model requires a higher density of dark matter at the center of galaxies, and because normal matter is attracted to dark matter, it should peak at the center too – something not seen in observations. However, fuzzy dark matter interacts with itself in such a way that it is more evenly spread, so when the normal matter is attracted to the dark matter it is more spread out over large scales, which does match observations.
The researchers tested if the fuzzy dark matter model works for actual galaxies. Using NASA’s Chandra, they studied the emission of hot gases in 13 galaxies, which is a good tracer of dark matter. Based on data gathered by Chandra, the team estimated how much dark matter would be present in each galaxy, and how it's spread out from the center. Models in which fuzzy dark matter particles had the same amount of energy did not work at all, but if the fuzzy dark matter particles were allowed to be “excited” then they were able to match the observations just as well or better than cold dark matter.
While intriguing, this is only the first step for a potential alternative to cold dark matter. There’s a long way to go before this could be considered a serious competitor.