While we can't see dark matter, it's thought to make up a large part of our universe's mass. Gemini Image Gallery

When researchers at Yale University said in March 2018 that they discovered a galaxy containing no dark matter, their work was met with a mixture of praise and criticism.

Much of the criticism of the discovery was that it contradicted dark-matter theory.

However, the scientists recently made an exciting discovery: They found yet another galaxy that appears to contain only "normal" matter.

When researchers at Yale University presented their observations of galaxy NGC 1052-DF2 in March 2018, their work was met with a mixture of praise and criticism.

Their research suggested that the unusual galaxy contained little to no dark matter. The idea contradicted dark-matter theory, which was part of the reason it drew so much attention.

However, a team led by Pieter van Dokkum, the Sol Goldman Family Professor of Astronomy at Yale, said last month that it discovered yet another galaxy without any dark matter - a discovery that supports its initial observations that dark matter is separable from galaxies.

These findings may require us to rethink how galaxies are formed.

While we can't see dark matter, it's thought to make up a large part of our universe's mass. Our galaxies consist of dark matter as well as "normal" matter, like stars and planets, for example.

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The scientists' discovery of galaxies with little to no dark matter is unprecedented and somewhat surprising.

"The fact that we're seeing something that's just completely new is what's so fascinating," said Shany Danieli, who first spotted the galaxy two years ago, according to Keck Observatory. "No one knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object, whether it's a planet, a star, or a galaxy, that no one knew about or even thought about."

The researchers followed the movements of 10 star clusters, allowing them to ascertain the mass of each galaxy. They found that the galaxies contained only as much mass as the stars would normally have had, meaning that most likely there was probably only normal matter.

The second galaxy found to be absent of dark matter was dubbed NGC 1052-DF4 - a discovery that's just as exciting for the researchers as that of DF2.

"This means the chances of finding more of these galaxies are now higher than we previously thought," Dokkum said, according to Keck Observatory. "Since we have no good ideas for how these galaxies were formed, I hope these discoveries will encourage more scientists to work on this puzzle."

Until recently, it was thought that galaxies couldn't form without dark matter. But Risa Wechsler, a Stanford University physics professor, told National Geographic last month that "we need to rethink what a galaxy is."

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According to Keck Observatory, both DF2 and DF4 belong "to a relatively new class of galaxies called ultra-diffuse galaxies." They're roughly the same size as the Milky Way but have far fewer stars, "making them appear fluffy and translucent, therefore difficult to observe."

Despite the lack of dark matter in these galaxies, the researchers say their existence bolsters the dark-matter theory, backing up the idea that dark matter is independent of "normal" matter.

The researchers said they want to continue exploring new galaxies. "Our hope is that this will take us one step further in understanding one of the biggest mysteries in our universe - the nature of dark matter," Danieli said.

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