Dark Matter Should Be Everywhere, but This Mysterious Galaxy Has None
Dark matter is thought to make up some 85 percent of the mass of the universe. Weird and invisible, dark matter has not been directly observed—no one is sure what it is, exactly—but it is key to explaining the movement of galaxies in space.
Some galaxies like our own are predicted to have about 30 times more dark matter than normal matter, whereas dwarf galaxies can have up to 400 times more. Now, however, researchers have found a galaxy that seems to have no dark matter at all.
Lying some 63 million light-years from the Milky Way, the elliptical galaxy NGC1052-DF2 seems to be completely made up of normal matter, defying all expectations.
Scientists can figure out how much mass there is in a galaxy by tracking how fast things inside move, Pieter van Dokkum, one the authors of a new research paper published in Nature, told Newsweek. For example, if the sun was four times as massive, the Earth would need to move twice as quickly to stay in the same orbit. How fast the Earth moves around the sun can tell you the star's mass, he explained.
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The researchers approached this distant galaxy in the same way. They followed the motion of 10 star clusters to work out how much mass the galaxy had. The difference between this number and the apparent mass of visible stars helps scientists estimate how much galactic mass is dark.
To their surprise, they found it had none. The visible mass was essentially the same as the total mass of the galaxy. It was made up of thoroughly normal matter. "If there was a lot of dark matter, the star clusters would move a lot faster than they do," added van Dokkum, who is a professor of astronomy at Yale University.
The discovery, van Dokkum explained, "was completely unexpected." In fact, the research team had previously looked at galaxies of the same class as NGC1052-DF2 and found they were made up almost entirely of dark matter. "So finding the opposite, namely an absence of dark matter, really came out of the blue for us," he said.
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Dark matter, it seems, might be spread out across the universe more unevenly than scientists had thought, van Dokkum explained.
Finding a total lack of dark matter might also, paradoxically, give scientists evidence for the existence of the strange stuff. Some theories say that dark matter is only an illusion caused by a failure to understand the way gravity works on the grand scale of the universe. "In those theories, every galaxy should show a dark matter signature, as it's not due to dark matter at all but due to the laws of physics," van Dokkum explained.
Learning more about the distribution of dark matter might one day help scientists learn more about its nature—for example, what kind of particle it is. But, van Dokkum said, "We're not there yet."
