Too little dark matter
💥 Four galaxies are challenging one of astronomy's biggest ideas.
with Ethan Siegel • July 4, 2026
Greetings readers,
Here in the United States today, we’re celebrating the country’s 250th anniversary in true American fashion: with loud bangs, glorious light shows, deliciously grilled foods, and frozen treats to help us beat the summer heat. But that’s nothing compared to the fireworks that we experienced long ago: when an asteroid struck our planet and drove our predecessors, the dinosaurs, to extinction. The dinosaurs, however, didn’t have telescopes, satellites, or the ability to predict what would be potentially hazardous far into the future. If we didn’t have those abilities either, and a similar event were headed our way, how much lead time would we have between when we could first detect that hazard and when the actual catastrophic impact occurred?
On the darker side of things, scientists just announced the discovery of DF9: the fourth allegedly dark matter-free galaxy of all. With four examples, you might think that we’ve now demonstrated the existence of galaxies that truly lack dark matter, but many are far from convinced. There’s a simple way out: Perhaps we’ve just mismeasured the distance to these few objects? You might think it’s easy to calculate the number of stars in the Universe: Just count the stars in the Milky Way and multiply by the number of galaxies we have across the cosmos. But if you do that, you’ll greatly overestimate the total number of stars. Here’s how to get it right, instead.
There’s a uniquely strong signature of the element silicon coming from a relatively nearby galaxy, NGC 1277, which hasn’t formed stars in over 10 billion years. Could that silicon be a compelling hint that will someday lead us to an understanding of the first stars? And finally, an ultra-massive galaxy cluster was found earlier in cosmic history than ever before, and it’s even been caught gravitationally lensing a set of even more distant objects. Here’s what that implies for dark matter. As always, it's a pleasure to share another week of awe, wonder, and joy of learning about the Universe with you, and we’ll be back with even more next week!
All the best,
Ethan
DARK MATTER DETECTIVE
Is it finally time to take dark matter-free galaxies seriously?
Less than a decade ago, astronomers identified the first seemingly dark matter-free galaxy: the ultra-diffuse dwarf galaxy NGC 1052-DF2. The discovery sparked enormous controversy, as different distance measurements led to dramatically different estimates of its dark matter content. Today, four galaxies appear to lack dark matter, but three of them lie on the outskirts of the same galaxy group. Are these truly dark matter-free galaxies, or have we been getting their distances wrong all along?
COSMIC TIME CAPSULE
Could nearby relic galaxy NGC 1277 lead to the first stars?
Located in the Perseus Cluster, NGC 1277 is the nearest known example of a massive, “red-and-dead” galaxy, containing only ancient stars and no gas for new star formation. Astronomers typically search for the Universe’s first stars by looking to its most distant reaches, hoping to catch them during their brief lifetimes. But what if the evidence of those first stars has survived much closer to home? NGC 1277, just 240 million light-years away, may hold the answer.
ASK ETHAN
Ask Ethan: Could we spot an extinction-level asteroid?
We know a great deal about the asteroid impact that triggered Earth's fifth great mass extinction: a massive asteroid struck the planet, carved out the Chicxulub crater, and wiped out roughly three-quarters of all species. But with only the naked eye, there would have been little warning before it arrived. If a similar object were headed for Earth today, how much advance notice would we get, and when would we know it was on a collision course? The answer reveals just how important planetary defense has become.
If you have a burning question about the Universe,
email startswithabang@gmail.com!
COUNTING STARS
How to not overestimate the number of stars in the Universe
If you look across the observable Universe, stretching 46.1 billion light-years in every direction, you’ll find trillions of galaxies. Since the Milky Way contains a few hundred billion stars, it might seem reasonable to estimate the total number of stars by simply multiplying the two. But neither our galaxy nor our Sun is typical. Here’s a better estimate of how many stars fill the Universe, and how different they are from the ones we know best.
A WARPED VIEW
The most distant lensing cluster reveals a dark matter surprise
In theory, the first mature galaxy clusters shouldn’t appear until about 3 billion years after the Big Bang. While Hubble previously imaged XLSSC 122, the most massive early cluster yet discovered, JWST reveals it in stunning new detail, including blue arcs created by strong gravitational lensing: the most distant lensing cluster ever found. Those observations reveal something unexpected: its dark matter is far more concentrated than in nearby galaxy clusters. Here’s what that surprising discovery could mean.
Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.
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