Astronomers Have Discovered A Galaxy With No Dark Matter And It's A Huge Deal

A Hubble Space Telescope image of the galaxy NGC1052-DF2. It is so ghostly that you can see distant galaxies through it. P. van Dokkum; R. Abraham; STScI, Space Telescope Science Institute

For the first time ever, an international team of astronomers has discovered a galaxy lacking dark matter, the hypothetical and mysterious substance that's expected to outweigh regular matter – which makes us and everything we can see in the universe – five to one. 

The discovery, published in Nature, is monumental. Dark matter is currently the best explanation for how galaxies are distributed in the universe and how they behave. Finding a galaxy with no dark matter challenges this assumption. We thought that all galaxies formed inside dark matter blobs, or halos, but it seems there is at least one exception to the rule.

“It tells us something about dark matter that we didn’t know, namely that it is separable from galaxies," Professor Pieter van Dokkum from Yale University told IFLScience. "Wherever we saw a galaxy before, we also saw dark matter. This suggested a direct coupling of where the dark matter is and where the galaxies are. The whole scaffolding upon which the structure in the universe is built is essentially dark matter. The galaxies are the froth that floats on this sea of dark matter.”

The object, known as NGC1052-DF2, was first recognized as a peculiar galaxy using the Dragonfly Telephoto Array, a groundbreaking instrument dedicated to searching for particularly faint objects. The team used 10 bright star clusters to work out its mass distribution and discovered that the mass of the galaxy was essentially the same as the mass of the stars in the galaxy.

The team followed up these observations with the Hubble Space Telescope which showed them a stunning object, an almost ghost-like galaxy. They also discovered that although DF2 is roughly the same size as the Milky Way (about 100,000 light-years across) it has 200 times fewer stars than our own galaxy. Estimates suggest that there should be 400 times more dark matter in DF2.

“Finding a galaxy without dark matter raises questions. Why is this? Is it because dark matter clumps in a different way than we thought in the early universe? Is it because we don’t really understand how dark matter behaves for these low-mass galaxies? Or is this just some weird object that formed in a way that is incredibly rare? We don’t know the answers yet. We don’t know how common this kind of galaxy is,” Professor van Dokkum explained.

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