Dark matter is just that – it's dark.
This mysterious matter spreads across the universe and does not interact with light, making it extremely difficult to spot. Scientists have not directly measured dark matter, but it is thought to make up about 27 percent of the universe (compared to just 5 percent of known matter like stars and galaxies). Without dark matter, theoretical models of our universe simply would not add up.
But the idea that dark matter is a necessary component of galaxy formation is being tested, reports Will Dunham of Reuters. Astronomers have found a distant galaxy that seems to contain no or almost no dark matter.
Researchers discovered the sparse translucent galaxy NGC1
The analysis shows that the ultra-diffuse DF2 is about 6.5 billion light-years away and about the same size as our own Milky Way, but contains 200 times fewer stars. T discovered 10 compact groups of these stars, also known as globular clusters, within the galaxy, according to a press release by Gemini . But these clusters moved much more slowly than the scientists' models predicted, suggesting that the system had less mass than would be expected if dark matter were present. The researchers elaborate their findings in a study published in the journal Nature
: "If there's any dark matter, it's very small," says Pieter van Dokkum of Yale, chief of the research team "The stars in the galaxy can be responsible for the entire mass, and there seems to be no room for dark matter."
DF2 exaggerates current theories about how galaxies form that predict the heaviness of dark matter are necessary for early galaxies to be related. "It's like taking a galaxy and having just the star wreath and globular clusters and somehow forgetting to do everything else," says van Dokkum of DF2 in the news release. "There is no theory that foretells these types of galaxies, the galaxy is a complete mystery because it's all weird about how to actually shape one of these things is completely unknown."
While discovering the existence of dark matter seems to refute – or implies that it is unnecessary for the formation of galaxies – it could do exactly the opposite. The discovery of DF2 represents an important step in confirming the existence of dark matter.
As Nola Taylor Redd of Space.com reports, theories that deny the existence of dark matter argue that the gravitational effects attributed by scientists to the substance , which could be explained optimizes what we know about gravity and astrophysics. "In these theories, dark matter is not real, but an illusion caused by our lack of knowledge about gravity on a large scale," says van Dokkum. "If that's the case, every galaxy should have a Dark Matter signature – you can not turn that on or off in these models." But DF2 does not seem to have dark matter, suggesting that the thing is not just an "illusion" or a glitch in the equation.
As reported by Ryan F. Mandelbaum at Gizmodo the same astronomical team found another ultra-diffuse galaxy called Dragonfly 44 in 2016, which seems to consist of 99.99 percent of dark matter, as weird and variable these diffuse galaxies can be.
So, if dark matter was not there to help DF2, how did that happen? As Redd reports, the researchers believe it could have formed when two other galaxies merged. The diffuse DF2 could consist of gas and boulders that were removed during the merger. Another possibility is that interstellar winds collected enough material to coalesce the low-mass galaxy.
According to the press release, the team has identified 23 other diffuse galaxies, including three with similar properties as DF2, and hope to start exploring them in detail soon to extract their secrets.
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