Dark matter is one of the main elements present in today’s galaxies, but has it always been like that? A new study published in the journal Nature suggests that only played a minor role in early galaxies formed at the dawn of the universe.
Dark matter in today’s universe
About a quarter of the universe is composed of dark matter. The rest is regular matter, which makes up only a small percentage, and dark energy. Dark matter cannot be seen, as it does not emit, absorb, or reflect light. However, scientists were able to discover it when they noticed the movement of galaxies as if they were influenced by gravity.
Dark matter and regular matter interact via gravity. Also, their interaction is important in the rotation speed of spiral galaxies. The presence of the two types of matter explains why spiral galaxies rotate more quickly on their outer part. This would not have been possible if only regular matter were present in the universe.
Dark matter in the early universe
Researchers noticed that the outer parts of early galaxies spin slower than their central regions. This characteristic is not met in today’s galaxies. The explanation would be that dark matter was present in smaller percentages in the early universe and it played a less important role then than it does today.
Reinhard Genzel and his colleagues from the Max Planck Institute for Extraterrestrial Physics in Germany looked at the rotation of six galaxies from the early universe. They used the Very Large Telescope and witnessed them as they were at the peak of galaxy formation, namely 10 billion years ago.
As compared to modern spirals, these distant galaxies rotated more slowly in their outside regions than in the center. This could only suggest that they were dominated by normal matter and dark matter was less present than it is today in the universe.
The bright region where half of the galaxy light is emanated, or the effective radius, of today’s galaxies is between 50 and 80 percent dark matter. More than half of the distant galaxies analyzed had 10 percent or less dark matter in their effective radius. Researchers found a possible explanation for this.
Three or four billion years after the Big Bang, the dark matter halos that surrounded galaxies were more large and more spread out. It took them several billions of years to condense into their form of today, which causes the high speed at which spiral galaxies spin.