Dark Matter Today
Dark matter makes up about a quarter of the universe, which is far greater than ordinary matter that only makes up about 5 percent. The rest is composed of dark energy.
Although dark matter is invisible because it does not absorb, emit, or reflect light, it was discovered after scientists noticed that galaxies move as if they are influenced by gravity.
Dark matter interacts with normal matter through gravity. It is also responsible for the rotation speeds of spiral galaxies.Their presence can explain why outer parts of spiral galaxies rotate more quickly than would be expected if only the visible normal matter is present.
Dark Matter In The Early Universe
In the new study of early galaxies published in journal Nature, researchers observed that the galaxies' outer regions spin slower than the central regions, which is unlike the characteristics of galaxies in modern universe. This means that there is less dark matter present than expected and that dark matter played a minor role in the early universe.
Reinhard Genzel, from the Max Planck Institute for Extraterrestrial Physics in Germany, and colleagues used instruments at the Very Large Telescope in Chile to measure the rotation of six massive galaxies from the early universe during the peak of galaxy formation about 10 billion years ago.
The researchers analyzed the formation of the galaxies to determine how much dark matter they possessed. The results revealed that unlike the spiral galaxies in the modern universe, the outer regions of the distant galaxies appear to rotate more slowly compared with regions that are closer to their core, which suggests that these is less dark matter in these galaxies.
Genzel said that most of the early massive galaxies were found to be dominated by normal matter.
The effective radius, or the bright region where half of the light of galaxies emanate, of the Milky Way and other typical galaxies in the modern day universe is between 50 and 80 percent dark matter. In half of the galaxies in the early universe that the researchers observed, dark matter only composed 10 percent and even less of the effective radius of the early galaxies.
The researchers suggest that between 3 and 4 billion years after the Big Bang, the gas present in the galaxies condensed into flat, rotating disks and the dark matter halos that surround them were much larger and more spread out.
It likely took billions of years longer for the condensation of dark matter to occur, which can help explain why its dominant effect is only observed on the velocity at which galaxy disks rotate today. The explanation does well with observations that show early galaxies were much more compact and gas-rich compared with today's galaxies.
"The observations suggest that baryons in the early Universe efficiently condensed at the centres of dark matter halos when gas fractions were high, and dark matter was less concentrated," researchers wrote in their study.