Recap: dark matter and galaxy rotation

In the late 1970s, astronomers Vera Rubin and Kent Ford of the Carnegie Institution observed our most well-known galactic neighbor: the Andromeda galaxy. And when they did, they discovered that the galaxy wasn't rotating the way they expected. In our solar system, the planets rotate around the Sun at different rates. Close-in Mercury moves much quicker than distant Neptune. However, in Andromeda, the visible stuff on the galaxy's outer rim moves just as fast as the stuff orbiting near the galaxy's core.

Rubin and Ford were perplexed. It meant the Andromeda galaxy must be saturated with huge amounts of invisible matter, stretching far from the galaxy's center. Ultimately, their discovery turned out to be the first direct evidence for dark matter.

Astronomers infer the existence of dark matter largely based on the fact that the rotation curves of galaxy's are not what you would expect without some form of hidden mass spread throughout the entire galaxy. In this simulation, the galaxy on the left shows what rotation would look like without the effects of dark matter, while the right shows rotation with dark matter. Note how the stars and gas on the outside of the right galaxy are spiraling much faster than those in the left galaxy.



Ingo Berg/Wikimedia Commons

Over the ensuing decades, astronomers realized that every galaxy seemed to be chock-full of dark matter, a substance that doesn't interact with regular matter or light, except through the force of gravity. Then, in 2018, researchers led by Pieter van Dokkum of Yale University found a bizarre, ghostly galaxy named NGC 1052-DF2 that seemed to contain very little, if any, dark matter.

"We thought that every galaxy had dark matter and that dark matter is how a galaxy begins," van Dokkum said in a press release after the discovery. "This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real: it has its own separate existence apart from other components of galaxies."

Just a few months later, van Dokkum and his team uncovered a second galaxy without any appreciable dark matter: NGC 1052-DF4. And like NGC 1052-DF2, this ultra-diffuse galaxy raised a lot of eyebrows in the astronomical community.

One critic was astronomer Ignacio Trujillo of the Instituto de Astrofisica de Canarias in Spain.

"Something that caught my attention very early on was the fact that the galaxy [DF2] was not only anomalous for not having dark matter, but also for having an extraordinarily bright population of globular clusters," Trujillo told Astronomy. "I remember thinking: 'Two anomalies at the same time really looks odd.'"