A strange new kind of galaxy has no dark matter, and we don't know why

Dark matter usually makes up most of the mass of a galaxy - but this strange galaxy has none.

Dark matter is usually present in galaxies - except when it isn't

Dr. Roberto Abraham (left) and his team pose with one half of the Dragonfly Telephoto Array that they've used to discover ultra-diffuse galaxies. (Dunlap Institute)
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Discovering ultra-diffuse galaxies

Dr. Roberto Abraham was looking forward to learning more about a new kind of galaxy he and his colleagues had discovered. But instead of finding answers, they're left with more questions.

Using an innovative new ground-based telescope they'd designed called the Dragonfly Telephoto Array, they'd discovered galaxies too dim to be easily detected by other instruments, called "ultra-diffuse galaxies." 

These were very different from our well defined spiral-armed Milky Way galaxy. They were big, but had less than a hundredth the number of stars of our galaxy. According to Abraham, "they look like spherical, almost translucent, kind of ghostly clouds." 

Where dark matter is

Two years ago Dr. Abraham, who's a professor of astronomy and astrophysics at the University of Toronto, had discovered that ultra-diffuse galaxies they were finding had vast amounts of dark matter associated with them.

Despite their paucity of stars, measurements of their gravitational pull indicated that they were extremely massive, and so had to be extremely rich in invisible, nearly undetectable dark matter. 

They thought this would make them perfect natural laboratories for understanding more about the mysterious dark matter.

Astronomers have found a galaxy that's left them scratching their heads: it lacks what they had believed was the main ingredient for galaxy formation. (Pieter von Dokkum)
Where dark matter isn't

A great candidate for this kind of study was a new ultra-diffuse galaxy they'd found that was relatively close to us, in cosmic terms, called  NGC1052-DF2. Its proximity would make studying the subtle effects of its dark matter a little easier.

However, when they looked for signs of dark matter in this galaxy they found, to their surprise, it had none. According to Abraham, it was deeply surprising to find apparently similar looking objects — these diffuse galaxies — that were so different in their dark matter. 

To this point they're only guessing on why this is. "The only explanation I can offer is philosophical," says Abraham. 

"The universe is this deeply mysterious and wonderful place, and any time anyone can figure out a way to look at it in a way that nobody has ever looked before, it's going to reward you with surprises."