There are few mysteries in the universe that are as mind-boggling as dark matter, the invisible force that is thought to hold entire galaxies together. Since dark matter doesn’t emit or reflect light, it can’t be observed directly, but that’s not to say there’s no evidence of dark matter out there; its influence can be seen just outside of many galaxies, where long strands of stars called stellar streams lie.
Stellar streams, which orbit a host galaxy, aren’t perfectly uniform. They have spurs, gaps, kinks, wiggles — all irregularities that have been linked to dark matter’s gravitational sway. But a new study published in The Astrophysical Journal has found that dark matter isn’t responsible for all irregularities. Simulations suggest that these features can also be shaped by the host galaxy itself. With this distinction in mind, researchers may now be able to hone in on exactly which features dark matter has a hand in creating.
“Now that we can predict what the host galaxy does on its own, we can start isolating the part that dark matter is responsible for,” said study lead author Arpit Arora, a postdoctoral scholar in astronomy at the University of Washington, in a statement.
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Explore All OptionsTracking Down Dark Matter
Stellar streams are formed when a group of stars gets torn apart by a larger galaxy; as the stars orbit the galaxy, gravity stretches them into long, thin filaments.
The vast majority of known stellar streams are those surrounding the Milky Way. According to a 2025 study in New Astronomy Reviews, over 120 stellar streams have been reported to exist in the Milky Way. These streams have become some of the most promising sources in the search for dark matter.
“Dark matter makes up most of the mass in the universe and forms the scaffolding that galaxies grow on, but we still don't know what it is,” said study co-author Nora Shipp, an assistant professor of astronomy at the University of Washington. “The Milky Way is one of the best laboratories we have for figuring that out, and stellar streams are one of the sharpest tools inside it.”
The irregularities in stellar streams — where the band of stars may break or bend — are commonly thought to be caused by small clumps of dark matter called subhalos, but the subhalos are likely not alone in shaping the streams’ quirks.
Simulating Stellar Streams
The new study, rather than focus on dark matter’s impact, set out to understand how a host galaxy’s structure could cause irregularities in stellar streams. To find answers, astronomers simulated four Milky Way-sized galaxies without any dark matter clumps, and then threw roughly 15,000 stellar streams into the mix.
After five billion simulated years, nearly every stellar stream displayed irregularities. In the simulated galaxies, stars were spread somewhat unevenly in a way that reflected the structure of real galaxies, having areas of lesser and greater density. And as the simulated streams of stars passed through denser regions, they were contorted by gravity in various ways.
Irregularities All Over
Some streams in the simulations were totally torn apart, while others that orbited closer to the galactic core were immersed in dense regions, accumulating more irregularities. Overall, roughly three-quarters of streams developed complex features stemming from the host galaxy’s structure. Even streams that were relatively smooth still exhibited multiple gaps and a 10 to 25 percent width variation along their track.
Out of the 15,000 simulated streams, only 70 were left completely smooth after the five billion years.
While the study suggests that host galaxies play a significant role in shaping stellar streams, it doesn't count out the role of dark matter clumps. Including dark matter in future simulations could allow researchers to distinguish which irregularities are uniquely affected by subhalos. In addition, forthcoming observations from surveys like the Vera C. Rubin Observatory are expected to find even more stellar streams around the Milky Way, and potentially traces of dark matter along with them.
“Sadly, there's no magic wand to reveal the structure of dark matter,” said James Davenport, an assistant professor of astronomy at the University of Washington. “Streams are complex systems, but they're still the most interesting way to study the dark matter close to home.”
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in The Astrophysical Journal: No Stream Left Unscathed: The Imprint of a Host Galaxy
- This article references information from a study published in New Astronomy Reviews: Stellar streams in the Gaia era
