Triplet of ‘Ghostly’ Galaxies Hints at What Dark Matter Is — and Isn’t

The lack of dark matter in galaxies DF2, DF9, and DF4 (clockwise from left) was a shock to scientists, who speculate that they formed after an ancient collision.

DF2、DF9 和 DF4 星系（从左起顺时针方向）中缺乏暗物质令科学家感到震惊，他们推测这些星系是在古代碰撞后形成的。

About 65 million light-years from Earth, three galaxies don’t look quite right.

距离地球约 6500 万光年，三个星系看起来不太对劲。

They aren’t whirlpools bursting with light, but pallid objects dimly illuminated by a smattering of stars.

它们不是闪耀着光芒的漩涡，而是被零星星星照亮的苍白物体。

“They have this ghostly appearance,” said Michael Keim , an astrophysicist at Yale University.

耶鲁大学天体物理学家迈克尔·凯姆说：“它们有着幽灵般的外观。”

And this isn’t even the weirdest thing about them.

这还不是他们最奇怪的事情。

Dark matter is the invisible glue that keeps the largest structures in the cosmos together.

暗物质是将宇宙中最大的结构粘合在一起的看不见的粘合剂。

It draws ordinary, visible matter toward it, allowing stars to form and keeping the stelliferous limbs of galaxies from flying off.

它将普通的可见物质吸引向它，从而允许恒星形成并防止星系的恒星四肢飞走。

“Almost all galaxies are dominated by dark matter,” said Shyam Balaji , a theoretical physicist at King’s College London.

“几乎所有星系都由暗物质主导，”伦敦国王学院理论物理学家希亚姆·巴拉吉 (Shyam Balaji) 说。

But these three diffuse galaxies, known as DF2, DF4, and DF9, have left astronomers stunned.

但这三个弥漫的星系，即 DF2、DF4 和 DF9，却让天文学家感到震惊。

After using a suite of telescopes to estimate their masses — using the motions of their stars as a proxy — the researchers found that all three have little dark matter.

在使用一套望远镜估算它们的质量（以恒星的运动为代表）后，研究人员发现这三颗行星都几乎没有暗物质。

Instead, a pittance of ordinary matter alone must have forged their small selection of stars.

相反，仅仅少量的普通物质就一定形成了他们所选择的一小部分恒星。

How could these rule-breaking galaxies have formed?

这些违反规则的星系是如何形成的？

A paper published in June 2026 in The Astrophysical Journal offers a potential solution.

2026 年 6 月在《天体物理学杂志》上发表的一篇论文提供了一个潜在的解决方案。

Once upon a time, two regular galaxies collided.

很久以前，两个常规星系发生了碰撞。

Their ordinary matter — their gas and dust — swirled about and got clumped up.

它们的普通物质——气体和尘埃——旋转并凝结成块。

But the dark matter passed right through and continued on, like a specter passing through a wall.

但暗物质却直接穿过去，并继续前行，就像幽灵穿墙而过一样。

The cosmic pileup left behind three new galaxies, each bereft of dark matter, each left to struggle to form a new batch of stars.

宇宙堆积留下了三个新的星系，每个星系都失去了暗物质，每个星系都在努力形成新一批恒星。

It’s a convincing explanation for many experts — one that, crucially, may tell us something about the mystery of dark matter.

对于许多专家来说，这是一个令人信服的解释——最重要的是，它可能会告诉我们一些关于暗物质之谜的事情。

Is there a way of evolving the galaxies without dark matter in the first place?

有没有一种方法可以让星系在没有暗物质的情况下演化？

Our understanding says no.

我们的理解是不。

Chamkaur Ghag, University College London

Chamkaur Ghag，伦敦大学学院

Some scientists have suggested that dark matter, which has yet to be directly detected, may not exist.

一些科学家认为，尚未被直接探测到的暗物质可能并不存在。

Instead, they propose that a tweak to our understanding of the gravitational force might explain various features of the universe.

相反，他们提出调整我们对引力的理解可能会解释宇宙的各种特征。

These bizarre galaxies pour cold water on that idea.

这些奇异的星系给这个想法泼了一盆冷水。

“What this is showing is that dark matter is …

“这表明暗物质是......

not just a modification of gravity,” said Keim, who is an author of the new study.

不仅仅是重力的改变，”这项新研究的作者凯姆说。

If you segregate ordinary matter from dark matter, you’d expect that the dark matter–lacking galaxies would struggle to form stars — and that’s what DF2, DF4, and DF9 show.

如果你将普通物质与暗物质分开，你会认为缺乏暗物质的星系将难以形成恒星——这就是 DF2、DF4 和 DF9 所显示的。

That suggests, as many have suspected for some time, that dark matter is a physical thing.

正如许多人长期以来所怀疑的那样，这表明暗物质是一种物理物质。

“The absence of dark matter is, in a way, some evidence of dark matter, which is ironic,” Balaji said.

“在某种程度上，暗物质的缺失是暗物质存在的一些证据，这很讽刺，”巴拉吉说。

What, exactly, is dark matter?

暗物质到底是什么？

Ever since it was formally proposed by the astronomer Fritz Zwicky in the 1930s — and built upon in the 1970s by the astronomer Vera Rubin — cosmologists and astrophysicists have been putting forward their best candidates.

自从天文学家 Fritz Zwicky 在 20 世纪 30 年代正式提出并在 1970 年代由天文学家 Vera Rubin 进一步发展以来，宇宙学家和天体物理学家一直在提出他们最好的候选者。

Could it be a stealthy type of neutrino ?

它可能是一种隐形中微子吗？

Might it be so-called weakly interacting massive particles, or WIMPs ?

它可能是所谓的弱相互作用大质量粒子（WIMP）吗？

Some have even opined that it could be primordial black holes as small as individual atoms and as old as the universe itself.

有些人甚至认为它可能是像单个原子一样小、和宇宙本身一样古老的原始黑洞。

“We don’t know which dark matter candidate is the right one,” said Daniel Whiteson , a particle physicist at the University of California, Irvine.

“我们不知道哪种暗物质候选者是正确的，”加州大学欧文分校的粒子物理学家丹尼尔·怀特森说。

Whatever it is, it has yet to be detected.

不管它是什么，目前还没有被发现。

But “there’s a lot of independent lines of evidence that dark matter is matter.”

但“有很多独立的证据表明暗物质是物质。”

In 2018, scientists used observations from the Keck Observatory in Hawai‘i to estimate the mass of galaxy DF2.

2018 年，科学家利用夏威夷凯克天文台的观测结果估计了 DF2 星系的质量。

They discovered that it contained 400 times less dark matter than expected.

他们发现它所含的暗物质比预期少 400 倍。

WM Keck Observatory/Ethan Tweedie

WM 凯克天文台/Ethan Tweedie

Dark matter, which makes up 27% of the universe — and 85% of all matter —interacts with gravity.

暗物质占宇宙的 27%，占所有物质的 85%，与引力相互作用。

If it were down to the gravitational pull of ordinary matter alone, stars on the edges of rapidly spinning galaxies would escape into space.

如果仅仅依靠普通物质的引力，快速旋转星系边缘的恒星就会逃逸到太空中。

Ordinary matter alone would struggle to clump together tightly enough to form stars.

仅普通物质就难以紧密地聚集在一起形成恒星。

Dark matter also seems to warp space itself, bending light around objects in a process known as gravitational lensing.

暗物质似乎也会扭曲空间本身，在称为引力透镜的过程中使物体周围的光线弯曲。

“You wouldn’t get galaxies without dark matter,” said Chamkaur Ghag , a physicist at University College London.

伦敦大学学院的物理学家查姆考尔·加格（Chamkaur Ghag）说：“没有暗物质就不可能有星系。”

“Is there a way of evolving the galaxies without dark matter in the first place?

“有没有一种方法可以让星系在没有暗物质的情况下演化？

Our understanding says no.”

我们的理解是不可以。”

So when DF2 was discovered in 2018, astronomers didn’t quite know what to make of it.

因此，当 2018 年发现 DF2 时，天文学家还不太清楚如何理解它。

The galaxy is roughly the same size as the Milky Way, but with 200 times fewer stars, it’s considerably dimmer.

该星系的大小与银河系大致相同，但恒星数量少了 200 倍，因此亮度要低得多。

Suspicious, scientists used the Keck Observatory in Hawai‘i to track the motions of those stars to estimate its total mass.

出于怀疑，科学家们利用夏威夷的凯克天文台追踪这些恒星的运动，以估计其总质量。

They discovered that, even for such a star-deficient galaxy, it was lightweight: It seemed to contain, at most, 400 times less dark matter than they would expect, making DF2 the first dark matter–deficient galaxy ever spotted.

他们发现，即使对于这样一个缺乏恒星的星系，它的质量也很轻：它所含的暗物质似乎最多比他们预期的少 400 倍，这使得 DF2 成为有史以来发现的第一个缺乏暗物质的星系。

In 2019, they observed DF4 , another phantom galaxy with a dearth of dark matter.

2019年，他们观测到了另一个缺乏暗物质的幻影星系DF4。

And in 2026, scientists announced that a third, DF9, had joined the club.

2026 年，科学家宣布第三个 DF9 已加入该俱乐部。

It contains the mass of roughly 100 million suns, which is consistent with having only ordinary matter.

它的质量大约相当于一亿个太阳，这与只有普通物质是一致的。

If it were also packed with dark matter, it would be 100 times more massive.

如果它也充满暗物质，它的质量将是原来的 100 倍。

That all three gossamer galaxies existed along a line, as neighbors, didn’t escape the attention of astrophysicists, who began to coalesce around an origin story .

三个薄纱星系作为邻居沿着一条线存在，这一点并没有逃过天体物理学家的注意，他们开始围绕一个起源故事结合起来。

This image from the Hubble Space Telescope shows neighboring galaxies DF2, DF9, and DF4.

这张来自哈勃太空望远镜的图像显示了邻近的星系 DF2、DF9 和 DF4。

An ancient smash-up may have stripped away their dark matter, leaving only ordinary matter behind.

一次古老的碰撞可能已经剥离了它们的暗物质，只留下普通物质。

ESA/Hubble/NASA/Digitized Sky Survey 2/Acknowledgement: Davide de Martin

ESA/哈勃/NASA/数字化巡天 2/致谢：Davide de Martin

Dark matter is only known to act with the gravitational force; there’s no compelling evidence yet that it interacts with other fundamental forces, like electromagnetism.

目前已知暗物质仅通过引力作用。目前还没有令人信服的证据表明它与其他基本力（例如电磁力）相互作用。

If two typical galaxies met in a head-on collision, the dark matter would experience very little friction, or drag, and would keep on rocketing by.

如果两个典型的星系正面碰撞，暗物质将经历非常小的摩擦或阻力，并且会继续快速飞行。

But ordinary matter, which interacts using all four fundamental forces, would get all tangled up.

但是，使用所有四种基本力相互作用的普通物质会变得纠缠在一起。

“An analogy would be two knights jousting, where the knights and their lances collide and may be thrown to the ground in a heap, while their horses keep on running at full speed,” said Aaron Romanowsky , an astronomer at San José State University.

圣何塞州立大学天文学家亚伦·罗曼诺夫斯基（Aaron Romanowsky）说：“打个比方，两个骑士比武，骑士和他们的长矛相撞，可能会被扔到地上成堆，而他们的马却继续全速奔跑。”

Several independent experts buy the tale spun by Keim and his colleagues.

一些独立专家相信凯姆和他的同事编造的故事。

“It’s very convincing,” Whiteson said.

“这非常有说服力，”怀特森说。

It’s reminiscent of the Bullet Cluster , two massive collections of galaxies 3.8 billion light-years away that also collided eons ago.

它让人想起子弹星团，这是两个距离 38 亿光年的巨大星系集合，也在亿万年前发生了碰撞。

This destruction derby, which was recently observed by the James Webb Space Telescope, has a messy distribution of mass that only makes sense if dark matter and ordinary matter got torn asunder during the collision.

詹姆斯·韦伯太空望远镜最近观测到的这场毁灭德比具有混乱的质量分布，只有当暗物质和普通物质在碰撞过程中被撕裂时才有意义。

Although this kind of segregation can be seen with greater clarity in DF2, DF4, and DF9, not everyone agrees on how the separation took place.

尽管这种分离在 DF2、DF4 和 DF9 中可以更清晰地看到，但并非所有人都同意分离是如何发生的。

Kinwah Wu , a theoretical astrophysicist at University College London, noted that the triplet does not exhibit any of the fluid, wispy features that one would expect to see after a galactic collision.

伦敦大学学院的理论天体物理学家吴金华指出，三重态并没有表现出任何人们期望在星系碰撞后看到的流动、纤细的特征。

He suggests that, in this case, DF2, DF4, and DF9 were passing through a large group of massive galaxies.

他认为，在这种情况下，DF2、DF4 和 DF9 正在穿过一大群大质量星系。

This outsize rabble then stripped the triplet of all their dark matter during their ill-fated voyage.

然后，这群乌合之众在他们不幸的航行中剥夺了三重态的所有暗物质。

The Bullet Cluster is the result of a massive, ancient collision between two collections of galaxies.

子弹星团是两个星系团之间大规模、古老的碰撞的结果。

In it, intracluster gas (represented in pink) and dark matter (represented in blue) have separated.

其中，簇内气体（以粉色表示）和暗物质（以蓝色表示）已经分离。

Stacy McGaugh , an astronomer at Case Western Reserve University and a longtime proponent of modified-gravity theories, is more skeptical.

凯斯西储大学的天文学家、修正重力理论的长期支持者史黛西·麦高 (Stacy McGaugh) 持更为怀疑的态度。

“It’s not clear to me what the formation mechanism is,” he said.

“我不清楚形成机制是什么，”他说。

An ancient collision “is a plausible origin story, albeit an exceptional one.

一次古老的碰撞“是一个看似合理的起源故事，尽管是一个特殊的故事。

If that’s what’s happened, then the rest follows.” In other words: If these galaxies were created when their dark matter was stolen away, then it makes sense that dark matter is a physical substance.

如果事情确实如此，那么剩下的事情就会随之而来。”换句话说：如果这些星系是在暗物质被偷走时形成的，那么暗物质是一种物理物质就有意义了。

“But the question is: Is that what happened?” he continued.

“但问题是：事情真的是这样吗？”他继续说道。

“And I’m not convinced that that’s the only way that can happen.” Without definitively knowing their origin story, McGaugh doesn’t think it’s possible to say anything about the nature of dark matter itself.

“而且我不相信这是唯一可能发生的方法。”在不确定暗物质起源的情况下，麦高认为不可能对暗物质本身的性质做出任何解释。

Most experts, though, cannot see how DF2, DF4, and DF9 could exist without dark matter being a physical thing, even if the trio’s genesis can be debated.

然而，大多数专家无法理解 DF2、DF4 和 DF9 在没有暗物质的情况下如何存在，即使这三者的起源存在争议。

“It’s hard to explain how you’d get something like this evolving by itself without something like dark matter,” Ghag said.

加格说：“很难解释如何在没有像暗物质这样的东西的情况下让这样的东西自行进化。”

MOND says: What if there’s only visible matter, but its gravity works differently than we thought?

蒙德说：如果只有可见物质，但它的引力作用与我们想象的不同怎么办？

Daniel Whiteson, University of California, Irvine

丹尼尔·怀特森，加州大学欧文分校

That makes this galactic trio bad news for MOND, or modified Newtonian dynamics, an alternative to dark matter proposed in 1983 by the physicist Mordehai Milgrom .

这使得这个银河三重奏对于 MOND（即改良牛顿动力学）来说是个坏消息，MOND 是物理学家莫德海·米尔格罗姆 (Mordehai Milgrom) 于 1983 年提出的暗物质替代方案。

“MOND says: What if there’s only visible matter, but its gravity works differently than we thought?” Whiteson explained.

“蒙德说：如果只有可见物质，但它的引力作用与我们想象的不同怎么办？”怀特森解释道。

MOND implies that, when objects are experiencing a very weak gravitational pull — for example, when two stars are separated by extreme distances — the gravitational force doesn’t dwindle as much as scientists think.

MOND 意味着，当物体受到非常弱的引力时——例如，当两颗恒星相距极远时——引力的减弱程度并没有科学家想象的那么大。

That could explain why spinning galaxies can be held together without needing an extra source of gravity from dark matter, its proponents have argued.

它的支持者认为，这可以解释为什么旋转的星系不需要来自暗物质的额外引力源就可以聚集在一起。

Dark matter has long been the favored theory for cosmologists, as MOND “could never really explain everything,” Ghag said.

加格说，长期以来，暗物质一直是宇宙学家青睐的理论，因为 MOND“永远无法真正解释一切”。

He explained that MOND suffered a significant blow in 2015 when the U.S.-based Laser Interferometer Gravitational-Wave Observatory, or LIGO, detected gravitational waves — ripples in the fabric of the universe — for the first time.

他解释说，MOND 在 2015 年遭受了重大打击，当时美国的激光干涉仪引力波天文台 (LIGO) 首次探测到引力波（宇宙结构中的涟漪）。

Scientists found that these waves move at the speed of light in all directions over huge distances, which implies that gravity works in a consistent, unmodified way throughout the cosmos.

科学家发现，这些波以光速向各个方向移动很远的距离，这意味着引力在整个宇宙中以一致的、未经修改的方式起作用。

DF2, DF4, and DF9 are also more elegantly explained by dark matter than by attempts to adjust the mechanics of gravity.

DF2、DF4 和 DF9 也可以用暗物质来解释，而不是尝试调整重力力学。

“It’s easier to remove matter, cold dark matter, through something like a collision, than it is to turn off a law of nature,” Balaji said.

巴拉吉说：“通过碰撞之类的方式去除物质、冷暗物质，比关闭自然法则更容易。”

“I think it’s fine and healthy to have alternative theories and open debate,” Whiteson said.

“我认为拥有替代理论和公开辩论是好的和健康的，”怀特森说。

“But let’s also be frank about whether the theory fits the data.

“但我们也要坦率地讨论这个理论是否符合数据。

MOND does not.”

蒙德没有。”

The identity of dark matter still eludes us.

暗物质的身份仍然让我们困惑。

But these galaxies are arguably another nail in the coffin of its increasingly unpopular competitor.

但这些星系可以说是其日益不受欢迎的竞争对手的又一颗钉子。

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《Quanta》杂志对评论进行审核，以促进知情、实质性、文明的对话。

Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected.

辱骂、亵渎、自我推销、误导、语无伦次或偏离主题的评论将被拒绝。

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版主在正常工作时间（纽约时间）有工作人员，并且只能接受英文评论。