85 percent of the universe is made up of matter that we can’t see. Astronomers still don’t understand what all this invisible mass and energy is made of. However, researchers have now succeeded in indirectly mapping this elusive matter.
It is estimated that the universe consists of about 5% percent visible matter, 70% made up of dark energy (the still mysterious driving force behind the accelerated expanding of the universe), 25% dark matter.
However, dark matter has remained elusive to researchers. “The most important thing is that it interacts mainly with the rest of the universe by gravity,” says researcher Sownak Bose. “Today, there is even more compelling evidence for this by a phenomenon known as gravity lenses. In doing so, the gravity of dark matter deflects the light of an object further away from the observer.”
Another important feature of dark matter is that it is very cold. “This means that the particles did not move through the primordial universe at extremely high speeds, which is the case for photons and neutrinos, for example,” Bose continues. “If dark matter had been hot, the universe would not have looked like it does now. The rapid movements of a hot dark particle of matter make it more difficult for small galaxies to form. But we see these kinds of galaxies all around us.”
In addition, astronomers have shown that dark matter plays a crucial role in the formation of galaxies. But in many ways, we actually know better what dark matter isn’t than what it is. “It doesn’t interact with light and (almost) doesn’t interact with ordinary matter,” Bose said. And because of this, it’s very difficult to uncover the nature of dark matter. Attempts to detect dark matter directly have so far not been successful.
Using the power of supercomputers, a team has ‘zoomed in’ on the smallest clumps of dark matter in a virtual universe. But how can you capture something we can’t see? “We start by using observations of the real universe,” Bose explains when asked. “This way we can determine how much dark matter we need to put into our simulations. Fortunately, observations of cosmic background radiation from the Planck satellite help us with this. This cosmic background radiation can in some sense be seen as an ‘image’ of what our universe looked like when it was only 400,000 years old. It means we can figure out what the composition of the universe was in its younger years, including the amount of dark matter.”
When studying the structure of the halos, the researchers came across a surprising discovery: all halos of dark matter – whether large or small – have very similar, internal structures. They all have a greater density in the middle and then fan out to the edges becoming diffuser. It means that it is almost impossible to tell a difference between a dark matter halo of a massive galaxy and that of a small galaxy. “No matter how large or small these objects are, the structure remains the same,” Bose explains. “I think this is a very exciting find. It shows how ‘ordered’ even the most complex systems in our universe can be. In other words, gravity – the architect of our cosmos – has a number of fixed patterns along which dark matter divides. And they are, in my opinion, very neat.”
The question is whether we can ever fully unravel the mystery of dark matter. “I hope so!” says Bose. “There are so many brilliant and creative people who are working on the problem of dark matter from both a theoretical and experimental point of view. In the coming decades, there will also be a huge effort to build new ground and space instruments that will be able to gather an unprecedented amount of information about the nature of galaxies. And this will be the key that can open the door to discovering what dark matter actually is. I am hopeful that if and when this happens, dark matter will prove to be even more exotic than we could ever have imagined.”