Galaxy clusters collide; dark matter still a mystery

March 26, 2015

Dark matter is one of science's great mysteries. It makes up an enormous amount matter in the universe, it is invisible, and it does not correspond to anything in the realm of our experience. Different theories compete for an explanation, but so far none of them has prevailed. In a collaborative study between École Polytechnique Fédérale de Lausanne (EPFL) and the University of Edinburgh, scientists have studied how dark matter behaves when galaxy clusters collide with each other over billions of years. Published in Science, their findings challenge at least one major theory on dark matter.

Dark matter and galaxy clusters

Although it accounts for 90% of all matter in the universe and more than a quarter of its energy, we know very little about dark matter. One major idea among astronomers is that dark matter consists of a new subatomic particle that we haven't discovered yet. More exotic theories want dark matter to be a quantum defect from the birth of the universe, extra-dimensional mass, and even a modified form of gravity.

What we do know is that dark matter interacts with cosmic structures through gravity, shaping and molding them. For example, dark matter bends light that passes through it, distorting images of distant space objects. In addition, dark matter speeds up the motion of galaxies inside galaxy clusters, which are collections of hundreds of galaxies, containing literally astronomical amounts of stars, planets, and gases. Galaxy clusters are also 90% dark matter, which makes them ideal for studying it, especially when they collide into each other and force their respective dark matters to interact.

Peering into the dark

David Harvey at EPFL's Laboratory of Astrophysics studies galaxy cluster collisions to find clues about the nature of dark matter. Continuing his PhD work from the Royal Observatory at Edinburgh, he and his colleagues studied data from 72 galaxy cluster collisions. These cosmic crashes happen over the course of billions of years when galaxy clusters attract each other because of their gargantuan masses. When this happens, the dark matter in each galaxy cluster interacts with that of the other, offering a unique opportunity to study it.

Harvey's data came from the Chandra X-ray Space Observatory and the Hubble telescope, and included the famous Bullet Cluster collision, a collision of two galaxy clusters whose gas has been molded into the shape of a bullet. This particular collision is actually the best current evidence for the existence of dark matter.

The researchers analyzed the collision data to measure the change in momentum of dark matter when two galaxy clusters crashed into each other. Experiments on Earth, e.g. in the Large Hadron Collider, show us that when subatomic particles interact, they exchange momentum. Therefore, depending on what happened to the dark matter after the collision, the researchers could draw conclusions about its nature.

To test the theory that dark matter consists of particles, the study worked with two possible scenarios: Either the particles of the dark matter interacted frequently but exchanged little momentum, or they interacted rarely but exchanged a lot of momentum. In the first case, dark matter would slow down after the collision, because the frequent particle interactions would cause an additional "drag". In the second scenario, dark matter would tend to be scattered away and lost into space.

Surprisingly, the study discovered that dark matters in galaxy cluster collisions simply pass through each other. This implies that dark matter particles do not interact with themselves, which would have caused dark matter to slow down. Instead, it appears that while dark matter could interact "non-gravitationally" with visible matter, this is not the case when it interacts with itself.

More importantly, the study challenges the view that dark matter consists of proton-like particles - or perhaps any particles whatsoever. "We have now pushed the probability of two 'dark matter particles' interacting below the probability of two actual protons interacting, which means that dark matter is unlikely to consist of just 'dark-protons'," says David Harvey. "If it did, we would expect to see them 'bounce' off each other".
This work represents a collaboration of EPFL's Laboratory of Astrophysics with the Royal Observatory at the University of Edinburgh, the Institute for Computational Cosmology at Durham University, and the Mullard Space Science Laboratory at University College London.


Harvey D, Massey R, Kitching T, Taylor A, Tittley E. The non-gravitational interactions of dark matter in colliding galaxy clusters. Science 27 March 2015. DOI: 10.1126/science.1261381

Ecole Polytechnique Fédérale de Lausanne

Related Dark Matter Articles from Brightsurf:

Dark matter from the depths of the universe
Cataclysmic astrophysical events such as black hole mergers could release energy in unexpected forms.

Seeing dark matter in a new light
A small team of astronomers have found a new way to 'see' the elusive dark matter haloes that surround galaxies, with a new technique 10 times more precise than the previous-best method.

Holding up a mirror to a dark matter discrepancy
The universe's funhouse mirrors are revealing a difference between how dark matter behaves in theory and how it appears to act in reality.

Zooming in on dark matter
Cosmologists have zoomed in on the smallest clumps of dark matter in a virtual universe - which could help us to find the real thing in space.

Looking for dark matter with the universe's coldest material
A study in PRL reports on how researchers at ICFO have built a spinor BEC comagnetometer, an instrument for studying the axion, a hypothetical particle that may explain the mystery of dark matter.

Looking for dark matter
Dark matter is thought to exist as 'clumps' of tiny particles that pass through the earth, temporarily perturbing some fundamental constants.

New technique looks for dark matter traces in dark places
A new study by scientists at Lawrence Berkeley National Laboratory, UC Berkeley, and the University of Michigan -- published today in the journal Science - concludes that a possible dark matter-related explanation for a mysterious light signature in space is largely ruled out.

Researchers look for dark matter close to home
Eighty-five percent of the universe is composed of dark matter, but we don't know what, exactly, it is.

Galaxy formation simulated without dark matter
For the first time, researchers from the universities of Bonn and Strasbourg have simulated the formation of galaxies in a universe without dark matter.

Taking the temperature of dark matter
Warm, cold, just right? Physicists at UC Davis are using gravitational lensing to take the temperature of dark matter, the mysterious substance that makes up about a quarter of our universe.

Read More: Dark Matter News and Dark Matter Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to