New Image Shows How Dark Matter Ties Galaxies Together in a Giant Cosmic Web

dark matter
Composite image shows dark matter bridging galaxies billions of light years away. S. Epps & M. Hudson / University of Waterloo

The first image of a dark matter web connecting galaxies billions of light years away has been released, confirming that giant cosmic webs of this mysterious invisible substance help tie the universe together.

Dark matter makes up around 27 percent of the universe. Normal matter—the stuff we can see— makes up just five percent. The rest of the universe is dark energy, which is thought to be the driving force behind the universe's expansion.

We know dark matter exists because of the force it exerts on galaxies. Under observed gravitational forces, galaxies would end up rotating so fast they would be torn apart. Dark matter is thought to provide the additional force needed to hold them together.

But because we cannot see dark matter (it does not shine, absorb or reflect light), scientists do not know what it is made of or how it is distributed around the universe.

Scientists from the University of Waterloo, Canada, have gone some way to showing how it connects galaxies around the universe, creating a composite image of a dark matter web.

"For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together," astronomer Mike Hudson, said in a statement. "This image moves us beyond predictions to something we can see and measure."

Hudson is one of the authors of a study published in the Monthly Notices of the Royal Astronomical Society. The team used a technique called weak gravitational lensing to take images of more than 23,000 galaxy pairs located 4.5 billion light years away.

Weak gravitational lensing causes images of galaxies to become warped as a result of mass from a nearby object, like a planet or black hole. In this instance, the mass was dark matter. From this, they combined the images to make a map showing the dark matter and how it connects galaxies. Until now, this web of dark matter had been unobservable.

Their findings showed the web was strongest when galaxies were less than 40 million light years apart. Study co-author Seth Epps said: "By using this technique, we're not only able to see that these dark matter filaments in the universe exist, we're able to see the extent to which these filaments connect galaxies together."

The team added that the study serves as a basis for future studies into dark matter filaments, providing a "simple method of stacking filaments that can be applied to any weak-lensing data set ".

They added that projects like the Dark Energy Survey, an international effort to map hundreds of millions of galaxies, should make it possible to learn even more about what dark matter is: " With increase in statistical power, it will become possible to study the nature of filaments as a function of other properties such as halo mass, separation and redshift," they wrote.