New Map Sheds Light on Dark Matter That Binds Universe Together…


‘Dark matter is the most common stuff in the universe. A billion sub-atomic particles of dark matter pass through your outstretched hand every second, yet few if any of these ethereal particles might actually touch and rebound from your hand in your lifetime. Now, studies are beginning to shed some light on this mysterious substance.

Astronomers have known since the 1930s that there is more than just the visible universe. The Milky Way, the galaxy we live in, is spinning too fast to be held together by the gravity between its stars. If stars were all there is, we should have long ago been flung off our cosmic roundabout. Instead, our galaxy contains about six times more material of some kind than is accounted for by every atom of all the elements in the periodic table: material known as dark matter.

Dark matter is invisible and can be detected only though the effect its gravity has on things we can see such as passing rays of light – an effect known as gravitational lensing. This is like looking through an uneven pane of glass which, while transparent, is obvious because of the distortions it produces on objects seen through it. By calculating the extent of the distortion, it’s possible to work out the thickness of the glass.

The recently released Dark Energy Survey used gravitational lensing to generate a huge map of dark matter. As seen from the Blanco telescope in Chile, they saw a crisscrossing web of thick, dark matter filaments.

The recently released Dark Energy Survey used gravitational lensing to generate a huge map of dark matter. As seen from the Blanco telescope in Chile, they saw a crisscrossing web of thick, dark matter filaments.


A map of cosmic dark matter, as seen from the Earth. Colour represents dark matter, increasing towards red. Circles mark galaxies and galaxy clusters.(Source: Dark Energy Survey)

The dark matter web is the invisible scaffolding that underpins the entire visible universe. The scaffold formed very soon after the big bang, and its gravity began to pull in all the ordinary material of which stars, planets and people are then built. Indeed, the galaxies are all found along dark matter filaments, with clusters of up to a thousand galaxies located wherever filaments cross each other.

So now we know where dark matter is. To find out what it is, we need the Hubble Space Telescope to zoom right in to the dark matter map.

Light vs Dark

On Earth we use particle accelerators to find out what matter is made of, firing particles at each other with enormous energy and seeing what the collision produces. This has been the principle behind experiments from Lord Rutherford’s discovery of atoms in 1908, to mankind’s biggest experiment, the Large Hadron Collider at CERN. But we can’t capture dark matter, nor interact with it at all. Nature, however, provides the experiment for us: we can watch what happens when the dark matter around galaxies or clusters of galaxies smash into each other by chance.

Galaxies are made from three ingredients: stars, dark matter and swirling clouds of gas. When two galaxies collide – an event involving expanses of time and space and size that makes it hard to comprehend – individual stars almost always pass straight unscathed. They are pinpoints of matter separated by vast regions of empty void. Conversely, the clouds of gas smash into each other and are pulled by friction to a stop. Dark matter is expected to behave somewhere in between, and its trajectory out of a collision should reveal its properties.


Chart: Points representing visible stars and galaxies (left) as seen through effects of gravitational lensing (right). (Source: TallJimbo, CC BY-SA)

Astronomical Particle Colliders

In one well-studied collision known as the Bullet Cluster, dark matter appeared to whizz straight through the collision. As close as we can tell, dark matter kept pace with the stars, not measurably slowed by its ordeal. But interpreting only a single collision is difficult. We must reconstruct the 3D scene from just one viewpoint and one freeze-frame from a movie that lasts 100m years.

To crowd-source the full movie, we recently observed 72 high-speed collisions of the dark matter around galaxy clusters. We view some from the side, others head-on and each at a different stage in the crash. Reconstructing the statistical properties of a dark matter collision, we confirmed robustly that dark matter interacts very, very little with anything else.


The Cerro Tololo observatory, home of the Victor Blanco telescope used. (Source:David Walker, CC BY-SA)

The Dark Matter Drag Factor…

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