While the invisible substance known as dark matter is largely present in most modern galaxies, a new study published in the journal Science suggests that it was not nearly as common during the early universe.
Current estimates show that dark matter makes up roughly 84 percent of the total matter in the universe. Though the substance cannot be seen, researchers can figure out where it is by looking at its gravitational effects on visible matter.
This method helped astronomers from the Max Planck Institute determine that the matter has become more and more present over time. They found this by using the European Southern Observatory’s Very Large Telescope to examine six massive, star-forming galaxies that existed during the peak of galaxy formation 10 billion years ago. They then analyzed the rotation of those systems to see how much dark matter they contained.
The researchers found that modern galaxies, such as the Milky Way, have an “effective radius” — the bright region that supplies half their light — that is 50 to 80 percent dark matter. In contrast, half of the early galaxies looked at in the study had effective radii that were less than 10 percent dark matter.
While the results are surprising, the team believes there is a rational explanation for the trend. Early galaxies had continuous inflows from intergalactic pockets of gas and dust, which may have led them to have higher concentrations of normal matter. In contrast, present-day galaxies have lots of hot gas around them that prevents them from gathering intergalactic material.
In addition, the dark matter rings in early galaxies may initially have been more spread out and condensed over time.
“Surprisingly, the rotation velocities are not constant, but decrease further out in the galaxies,” said lead author Reinhard Genzel, a researcher at the Max Planck Institute, in a statement. “There are probably two causes for this. Firstly, most of these early massive galaxies are strongly dominated by normal matter, with dark matter playing a much smaller role than in the Local Universe. Secondly, these early discs were much more turbulent than the spiral galaxies we see in our cosmic neighborhood.”
Researchers plan to expand on this finding by conducting more research on smaller galaxies that existed throughout the early universe. They hope this will provide them with a better picture of how certain systems evolved, shed light on the way dark matter has changed over time, and even give rise to new theories about the mysterious substance.
“There are a number of people speculating that perhaps we are on the wrong track as far as the nature of dark matter is concerned,” Genzel added, according to Gizmodo. “Maybe it’s not a particle.”