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Dark matter particles may scatter against each other only when they hit the right energy, according to a new study published in the Physical Review Letters. This may explain why galaxies of different sizes take the shapes that they do.

Dark matter is a mysterious and unknown form of matter that comprises more than 80% of the matter in the Universe today – believed to be responsible for forming stars and galaxies by its gravitational pull, leading to our existence.

As the paper’s author Hitoshi Murayama, a University of California Berkeley Professor and Kavli Institute for the Physics and Mathematics of the Universe Principal Investigator, says: “Dark matter is actually our mom who gave birth to all of us. But we haven’t met her; somehow, we got separated at birth. Who is she? That is the question we want to know.”

Astronomers have already found dark matter does not seem to clump together as much as computer simulations suggest it should. If gravity is the only force that drives dark matter, only pulling and never pushing, then dark matter should become very dense towards the centre of galaxies. However, in some case- particularly small faint galaxies called dwarf spheroidals – dark matter does not seem to become as dense as expected toward their centres.

This puzzle could be solved if dark matter scatters with each other like billiard balls, allowing them to spread out more evenly after a collision.

One problem with this idea is that dark matter does seem to clump in bigger systems such as clusters of galaxies. What makes dark matter behave differently between dwarf spheroidals and clusters of galaxies? The international team of researchers from institutes in Japan, Germany and Austria, has developed an explanation that could solve this riddle, and finally reveal what dark matter is.



Chinese physicist Xiaoyong Chu, a postdoctoral researcher at the Austrian Academy of Sciences, explains: “If dark matter scatters with each other only at a low but very special speed, it can happen often in dwarf spheroidals where it is moving slowly, but it is rare in clusters of galaxies where it is moving fast. It needs to hit a resonance.”

Resonance is a common phenomenon we encounter every-day. For example, Murayama points out, to get it more oxygen out of a glass of wine so that it lets out more aroma and softens its taste, you need to swirl it at the right speed. Or on an old analogue radio, you turn the dial to find the right frequency to tune into your favourite station.



The team suspects this is precisely what dark matter is doing.

Murayama continues: “As far as we know, this is the simplest explanation to the puzzle. We are excited because we may know what dark matter is sometime soon.”

However, Colombian researcher Camilo Garcia Cely, a postdoctoral researcher at the Deutsches Elektronen-Synchrotron (DESY) in Germany indicates that the team was not initially convinced that such a simple idea would explain the data correctly.

Cely says: “First, we were a bit skeptical that this idea will explain the observational data; but once we tried it, it worked like a charm!”

The team believes it is no accident that dark matter can hit the exact right note.

Cely continues: “There are many other systems in nature that show similar accidents: in stars alpha particles hit a resonance of beryllium, which in turn hits a resonance of carbon, producing the building blocks that gave rise to life on Earth. A similar process happens for a subatomic particle called phi.”

Chu continues: “It may also be a sign that our world has more dimensions than we see. If a particle moves in extra dimensions, it has energy.



“For us who don’t see the extra dimension, we think the energy is actually a mass, thanks to Einstein’s E=mc2. Perhaps some particle moves twice as fast in an extra dimension, making its mass precisely twice as much as the mass of dark matter.”

The team’s next step will be to find observational data that backs their theory.

Murayama says: “If this is true, future and more detailed observation of different galaxies will reveal that scattering of dark matter indeed depends on its speed.”



He is leading a separate international group that intends to do precisely this using the under construction Prime Focus Spectrograph. The US$80 million instrument will be mounted on the Subaru telescope atop Mauna Kea on Big Island, Hawaii, which will be capable of measuring the speeds of thousands of stars in dwarf spheroidals.

Original research Physical Review Letters: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.071103

Main image: Astronomers observed that the dark matter does not seem to clump very much in small galaxies, but their density peaks sharply in bigger systems such as clusters of galaxies. It has been a puzzle why different systems behave differently ( Kavli IPMU – Kavli IPMU modified this figure based on the image credited by NASA, STScI)





















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