The vast majority of galactic mass remains imperceptible to our current observational methods.
Current cosmological frameworks for galactic evolution posit that galaxies originate within substantial congregations of dark matter, which function as gravitational frameworks, facilitating the aggregation of conventional matter.
Consequently, a recent observation pertaining to a galaxy situated 67 million light-years distant presents a scenario that is both perplexing and exceptionally significant.
This celestial entity, designated NGC 1052-DF9, is not the solitary instance, nor the second, but rather the third known galaxy whose observed dynamics can be accounted for without recourse to the concept of dark matter.
The initial two galaxies, identified as DF2 and DF4, are intrinsically linked to the same galactic stream where DF9 is now located, exhibiting a linear alignment akin to precious stones adorning a strand.
While the precise mechanisms by which these galaxies became devoid of dark matter are not yet fully understood, theoretical predictions had previously suggested that if this particular galactic alignment originated under atypical circumstances, other galaxies within its sequence might also exhibit a deficiency in dark matter.
DF9 remarkably corroborates this conjecture.
“The overwhelming majority of galaxies across the cosmos are characterized by a dominant dark matter component. However, DF2, DF4, and now DF9 represent remarkable deviations from this norm,” states astrophysicist Michael Keim from Yale University, who spearheaded the investigation.
“These findings offer some of the most robust empirical support to date for the hypothesis that these galaxies coalesced through a cataclysmic event that effectively segregated baryonic matter from dark matter.”
Dark matter is an enigma intrinsically interwoven with nearly every facet of the universe.
Its precise composition remains unknown, yet it appears to exert a profound influence on the cosmological architecture of our universe.
Its existence is inferred from the fact that, after accounting for all observable, or baryonic, matter – encompassing stars, planets, galaxies, black holes, dust, and gases – the available visible mass is insufficient to explain the gravitational forces we detect.
The undetected source of this gravitational influence is something that eludes direct observation. The sole discernible interaction it has with the baryonic universe is gravitational. This enigmatic substance is termed dark matter, and it permeates the cosmos, outmassing normal matter by an approximate ratio of five to one.
Given that most galaxies are enveloped within vast agglomerations of dark matter known as halos, including our own Milky Way, scientific consensus holds that it plays a pivotal role in galactic formation.
Therefore, when DF2 was documented in a 2018 publication, under the lead authorship of astrophysicist Pieter van Dokkum from Yale University, exhibiting a significantly lower dark matter content than anticipated, the astronomical community was astounded.
Subsequently, in 2019, van Dokkum and his research cohort unveiled another surprising revelation.
While a single anomaly could be dismissed as an aberration or an instrumental error, the discovery of DF4, an object possessing identical characteristics to DF2 and situated within the same spatial vicinity, presented a more compelling case.
In 2022, van Dokkum and his collaborators, including Keim, reported that DF2 and DF4 were components of a linear formation comprising approximately a dozen galaxies arranged in close proximity.
And, in 2025, a subsequent investigation led by Keim and van Dokkum indicated that the galaxies within this structured alignment exhibit synchronized trajectories through space.
The researchers posited that if these galaxies share a common origin, and if two of them appear to be devoid of dark matter, it is plausible that other entities within this formation might exhibit similar attributes.
Among the galaxies constituting this formation, DF9 emerged as the most congruent with DF2 and DF4, sharing comparable dimensions, luminosity, and a similar complement of star clusters. This made it an optimal candidate for verifying whether other members of the sequence might also lack dark matter.
Consequently, a more detailed examination was conducted, and it was during this phase that DF9 revealed its unique characteristics.
“An alignment of galaxies deficient in dark matter has never been previously observed,” remarks Keim.
“This discovery provides some of the most substantial substantiation to date that these galaxies originated via an extreme and hitherto unobserved process, thereby affording a rare new perspective on the fundamental nature of dark matter itself.”
While the exact nature of this developmental process remains unknown, the research team speculates that a specific type of celestial event, termed a “bullet dwarf collision,” might be responsible.
Envision two dwarf galaxies traversing interstellar space on a direct collision course. Upon impact, the stellar components largely pass through one another, navigating the vast expanses between them.
It is hypothesized that dark matter behaves in a comparable manner, implying that the dark matter halos of the two galaxies would also pass through each other with minimal interaction.
The stellar and dark matter components would largely emerge from the collision unimpeded and continue on their respective trajectories.
However, gaseous matter exhibits divergent behavior. Interstellar gas clouds within each galaxy would collide forcefully, decelerating and becoming stranded, remaining behind long after the galaxies themselves have passed.
This scenario results in a region characterized by an abundance of ordinary matter but a relative deficit of dark matter. According to the simulations generated by the research team, this residual gas could subsequently aggregate to form stars, which in turn might coalesce into nascent galaxies devoid of dark matter.
This outcome is exceptionally noteworthy and could contribute to scientists’ efforts to constrain the possible nature of dark matter. The research team intends to continue their investigation of this peculiar galactic formation to uncover further indicative evidence.
“The revelation,” observes van Dokkum, “furnishes persuasive corroboration that dark matter functions as a physical entity rather than an artifact of an alternative gravitational theory, especially at the scale of dwarf galaxies where such theories are most vigorously debated.”
