An Ancient Galaxy Collision Sheds Light on Dark Matter

Scientists think they might have explained the bizarre discovery of unusual galaxies free of dark matter.
Scientists think they might have explained the puzzle of galaxies free of dark matter.
Hubble image of the system that contains DF2 and DF4. Image: Pieter van Dokkum et al./ Nature 2022
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Scientists think they have reconstructed the epic origin story of a trail of puzzling galaxies that could shed light on the great mystery of dark matter, an unidentified substance that accounts for most matter in the universe, reports a new study. 

It’s a tale that includes an ancient cosmic crash, galaxies that are bizarrely free of dark matter, and the medical television drama House.


It all began a few years ago, when a team led by Pieter van Dokkum, who serves as Sol Goldman Professor of Astronomy at Yale University, discovered a distant dwarf galaxy, called DF2, that appears to be totally bereft of dark matter. Given that all galaxies are thought to contain dark matter from their very origins, DF2 is a total oddball that challenges traditional ideas about galactic evolution, while also offering a tantalizing test of current explanations for dark matter. Flummoxed, van Dokkum and his colleagues reported the discovery in a 2018 Nature paper that sent ripples through the astronomical community.

“We decided to go ahead and publish it while saying and admitting that we don't know how this happened, we don't know where this thing came from, we don't know how it formed, but here's why we think this thing doesn't have dark matter,” van Dokkum and his colleagues said in a call. 

“At the same time, we made pretty grand claims about the implications of the objects—namely, that the fact that it didn't have dark matter would imply that dark matter is real and is actually a thing that that galaxies can have or not,” he continued. “That, in turn, rules out a whole class of alternatives to dark matter.” 


The study sparked spirited debate and speculation, which motivated van Dokkum and his colleagues to gather more data in order to try to explain this important cosmic puzzle. The team has now returned with an even more exciting discovery: It appears that DF2 is part of a whole trail of dark-matter-free galaxies—including one called DF4, reported in 2019—that may have been formed in “a near head-on collision between two gas-rich galaxies” that occurred eight billion years ago and straight-up shook the dark matter out them, according to a study published on Wednesday in Nature

“We kept getting new data and kept checking what we had done,” said van Dokkum, who also led the new study. “So here we are: We have not two, but ten galaxies without dark matter,” which he called a “super-exciting explanation.”  

Van Dokkum credits University of Oxford astronomer Joseph Silk for initially fleshing out this scenario, named a “bullet-dwarf collision,” in a model published in 2019. The name derives from the Bullet Cluster, a pair of colliding galaxy clusters that is regarded as extremely important for understanding dark matter. 


This crash, which is located nearly four billion light years from Earth, has separated the enigmatic dark matter in the clusters from the ordinary matter that makes up more familiar objects, such as stars and gas. This separation occurs because the gaseous portions of the cluster, which are made of regular matter, exert pressure on each other as they collide, causing them to halt at the collision front, just like a fender bender. 

Dark matter, in contrast, appears to only interact gravitationally with regular matter, which means it keeps right on moving during a high-speed collision. Voila: Dark matter and gas in the cluster seem to separate like oil and water. 

The Bullet Cluster was once seen as an utterly unique laboratory for studying dark matter in relative isolation. But now, van Dokkum and his colleagues think they have witnessed a very similar process playing out on a much smaller scale in this trail of galaxies that includes DF2 and DF4, which is located about 72 million light years from Earth. 

In addition to drawing on Silk’s model and publicly available imaging data, Van Dokkum said he was inspired by the television show House, in which Hugh Laurie’s doctor character uses a technique called “differential diagnosis” to single out the ailments of his patients. In much the same way, the team wrote all of their data to try to come up with an overarching explanation that fit all the pieces.


Van Dokkum initially began with the assumption that it would be way too much of a coincidence for DF2 and DF4—two unprecedented dark-matter-free galaxies in the same cosmic region—to be unrelated. The researchers then noted that the galaxies were, in fact, spinning away from each other at high speeds. 

By rewinding their trajectories in time, the team realized that DF2 and DF4 could have been forged in an ancient collision that would have spilled out a trail of similar dark-matter-free objects. This line of galaxies would, in turn, be bookended by the remnants of the galaxies that crashed eight billion years ago, which since became completely dominated by the dark matter from the original bullet-dwarf collision—and that is exactly what the researchers think images of the system reveal.

“If there are two galaxies that are on this particular axis and still spinning away from each other, there must be other pieces as well, in particular the two galaxies at the edges of the trail,” van Dokkum explained. “Looking at that distribution of galaxies in the field, and then seeing this line jump out at you, was a huge moment. It's like: ‘Oh wow, this is actually real.”

Indeed, van Dokkum’s team has pinpointed what is likely the ultra-faint silhouette of one of these dark-matter-dominated progenitor galaxies. They hope to search for the other progenitor, and study the whole system further, with next-generation observatories, including the recently launched James Webb Space Telescope. This effort could not only confirm this tentative explanation for the weird galaxies, it could streamline the search for hypothetical dark matter particles, which is one of the most important questions in science.

“If we can pinpoint where dark matter is, versus the stars in those galaxies at the edge, then we have really, really major results,” van Dokkum said. “Then we know how the dark matter behaves and what kind of particle to look for.”