Astronomers have taken another step towards solving the puzzle of what dark matter might be made of.
Dark matter is believed to make up the majority of matter in the Universe and is different from the matter that makes up people, planets and stars, but no one knows what it is.
Physicists have debated if the mysterious substance exists as actual particles or if what they see in their observations is a result of deviations in the laws of gravity.
Now an international team of scientists, including Durham University, UK, may have moved closer to understanding if dark matter is made of particles.
The researchers looked at images of millions of distant galaxies to see how their light is distorted by the gravity of foreground galaxies – a process called gravitational lensing.
The lensing they observed was stronger than the stars in the galaxies should produce according to Albert Einstein’s famous theory of General Relativity.
This pointed to the potential existence of a large amount of invisible dark matter particles.
The question facing physicists is whether dark matter is responsible for this extra lensing strength or if it is caused by assuming the wrong gravitational fields for galaxies.
The latest research tested measurements against expectations of both modified gravity and dark matter theories.
The new measurements extend much further into the outskirts of galaxies than previous work. This gave the researchers an advantage in the effort to understand dark matter because this region contains very little ordinary matter, but dark matter is still apparently abundant.
While researchers say the influence of dark matter is the most likely cause of the extra lensing, they added that a modified gravity explanation for the light distortion could also not be definitively ruled out.
The findings are published in the journal Astronomy & Astrophysics.
Research co-author Dr Kyle Oman, of the Institute for Computational Cosmology, Durham University, said: “The images of distant galaxies are distorted by the gravity of intervening galaxies bending the light rays on their way to us.
“This gravitational lensing is stronger than Einstein’s theory of General Relativity predicts would be caused by the visible stars and gas in the intervening galaxies.
“This points either to the presence of a lot of additional, invisible matter, in which case we are seeing another manifestation of the dark matter phenomenon, or we need to modify or replace General Relativity as a theory of gravity.”
The team measured the expected and actual gravity from distortions in the images of millions of background galaxies located behind 259,000 isolated foreground galaxies, taken during the European Southern Observatory’s (ESO) Kilo-Degree Survey (KiDS). This allowed them to measure the average amount of extra gravity exerted by each foreground galaxy.
The researchers found that older, red elliptical galaxies exert more extra gravitational force than younger, blue spiral galaxies.
Dr Margot Brouwer, of the University of Groningen and the University of Amsterdam, the Netherlands, who led the study, said: “When we discovered that the additional amount of gravity exerted by the two types of galaxies differed significantly, we took this at first as a strong hint pointing to the existence of dark matter as a particle.”
However, the researchers say a difference in the amount of gas surrounding the blue galaxies compared to red galaxies might also make an explanation relying on a modification of the theory of gravity viable.
To make a definitive statement about the measured difference between the two types of galaxies the surrounding gas would need to be measured and this is impossible with current telescope technology.
The research team compared the gravity measurements with different models of the Universe including computer simulations where the existence of dark matter particles is assumed.
One of the computer simulations, called MICE, provided good predictions for the extra gravity, while a second, called BAHAMAS, predicted less extra gravity than was measured.
The researchers found this surprising, because the BAHAMAS simulation uses a more sophisticated set of recipes in its calculations and made much more realistic predictions for the dark matter content of the isolated galaxies, which the study focussed on, than MICE.
The scientists say that working out why MICE gave more accurate predictions for the gravitational lensing effect, despite the simulation’s other limitations, will be worth following up in a future study.
The researchers also looked at two models where the law of gravity is adjusted – the Emergent Gravity (EG) model of study co-author Professor Erik Verlinde, of the University of Amsterdam, and the so-called Modified Newtonian Dynamics (MOND). Both moderately under-predicted the observed lensing effect.
Dr Oman’s role in the research was funded by the European Research Council and the Netherlands Foundation for Scientific Research.