On the outskirts of the Fornax Cluster, astronomers have spotted something that shouldn't exist: two galaxies that appear to contain little or no dark matter at all. FCC 224 and FCC 240, studied by Maria Luísa Buzzo of Yale University and her team, represent a possible third example of one of the universe's strangest cosmic oddities—and they're challenging everything we thought we knew about how galaxies hold themselves together.
For over a decade, ultra-diffuse galaxies have sparked fierce debate among astronomers. These systems are roughly the size of the Milky Way but contain far fewer stars and much less mass overall. Some behave as expected, holding onto substantial quantities of dark matter in addition to their visible stars. But a handful seem to defy logic entirely. They appear to be missing the dark matter that should, by all accounts, dominate their gravitational structure. Until recently, only two such galaxies were known: NGC 1052-DF2 and NGC 1052-DF4, both in the NGC 1052 group, raising the question of whether these were cosmic flukes or part of a broader phenomenon.
When Buzzo's team turned the MUSE instrument on the Very Large Telescope toward FCC 224 and its companion galaxy FCC 240, they uncovered striking similarities to the original pair. Both galaxies show extremely low velocity dispersions—their stars and globular clusters are moving so sluggishly that gravity alone, without any dark matter halo, can explain the motion. The dynamical masses within their half-light radius match their stellar masses almost exactly, far below what typical dwarf galaxies should weigh. Both also host abnormally bright globular clusters with luminosity patterns that closely match DF2 and DF4, hinting at a shared origin story.
The age of these galaxies adds another crucial clue. FCC 224 and FCC 240 are almost exactly the same age—around 10 billion years old—and their globular clusters share the same age and metallicity as the diffuse stellar bodies. This means stars and clusters formed together in a single, intense burst of star formation. That timing matches one of the strongest predictions of the "bullet-dwarf" scenario, a collision hypothesis proposed by theorists including Jarred Silk. The idea is elegantly violent: when two dwarf galaxies collide at high speed, the catastrophic impact can separate stars from much of their dark matter, leaving behind a galaxy almost wholly devoid of the invisible material that typically binds galaxies together.
Yet FCC 224 and FCC 240 diverge from their NGC 1052 cousins in one significant way. DF2 and DF4 are separated by approximately 240 kiloparsecs and sit amid an extended trail of low-surface-brightness galaxies stretching more than two megaparsecs—debris from an ancient collision. In contrast, FCC 224 and FCC 240 are far closer, separated by only 75 kiloparsecs and moving toward each other at just 16 kilometers per second. Rather than forming a scattered trail of fragments, they appear to form a compact, long-lived bound pair held together by their mutual gravity.
This discovery, uploaded to the arXiv preprint server on May 22, opens new questions about how dark-matter-deficient galaxies form and persist. Whether FCC 224 and FCC 240 represent another collision aftermath or formed through an entirely different mechanism remains an open question—but their existence alone expands our understanding of the universe's possibilities.