In the vast dark universe, something silently drifts through space-time, influencing other objects while remaining almost invisible. Imagine a galaxy-sized object with very few—or even no—luminous stars. What could it be? A ghost hiding in the darkness? A dark galaxy? Or perhaps a ghost galaxy?
Over the last few days, the internet has been filled with new articles about ghost galaxies and dark matter. It seems like the perfect time to ask: What exactly are ghost galaxies, and why are astronomers so excited about them?
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Ghost galaxies are among the most fascinating discoveries in recent galaxy formation studies, especially from the COCO (Copernicus Complexio) project. Normally, dark matter halos act as nurseries for galaxies. Gas falls into these halos, cools down, and collapses to form stars. But ghost galaxies follow a very different story.
Their dark matter halos have a circular velocity below a critical value of about 30 km/s. At such low velocities, gas cannot cool efficiently. An atomic hydrogen cooling floor of roughly 10,000 K, together with the heating caused by cosmic reionization, prevents the gas from collapsing into new stars. As a result, the main galaxy never forms.
However, these halos are not completely empty.
As the halo grows, it merges with many smaller halos that were just massive enough to form a few stars before reionization shut down star formation. When these small systems merge, gravity tears them apart and scatters their stars throughout the larger halo.
The result is something remarkable: a faint cloud of stars surrounding an empty center. Instead of a bright galaxy with stars concentrated in the middle, there is only an isolated stellar halo with no central galaxy.
“Present-day systems formed in this way would be ‘ghostly’ isolated stellar halos with no central galaxy.” — Wang et al.
Interestingly, these ghost galaxies typically have a median stellar mass of about 3 × 10⁶ solar masses, similar to many ultra-diffuse galaxies. They are not simply failed galaxies. Instead, they are cosmic shells built from the remains of many smaller galaxies.
Why Do Some Galaxies Lose Their Dark Matter?
While some galaxies never develop a bright central galaxy, others begin as normal galaxies but gradually lose their dark matter.
Recent results from the NewHorizon simulations show that some dwarf galaxies become surprisingly poor in dark matter. They begin with the expected dark matter halos predicted by the standard cosmological model, but over billions of years they experience a process called sustained tidal stripping.
As these dwarf galaxies orbit very close to a much larger companion, the giant galaxy’s gravity slowly tears away their dark matter halos. Eventually, only the stars remain, exposing what looks like a naked stellar core.
The simulations divide these galaxies into two main groups.
Dark Matter-Poor Galaxies contain less dark matter than expected, but they still roughly follow the normal relationship between stellar mass and halo mass.
Dark Matter-Deficient Galaxies are much more extreme. Their dark matter content becomes so small that the ratio between halo mass and stellar mass drops below 10. At this stage, the galaxy has lost most of its original dark matter through repeated close encounters with its massive companion.
These Galaxies Are Rare—But Important
These unusual galaxies are not just theoretical curiosities. They represent a small but statistically important part of galaxy evolution.
Nearly 30% of dwarf galaxies show some level of dark matter loss due to tidal stripping.
Around 10% become highly dark matter-deficient, with halo-to-stellar mass ratios below 10.
Ghost galaxies are even rarer. Simulations suggest they form in only about 5% of halos with masses around 4 × 10⁹ solar masses.
These galaxies also do not survive forever. About 70% of dark matter-deficient dwarf galaxies eventually merge with their larger companions within roughly 3.5 billion years.
Because new dwarf galaxies are constantly falling into larger systems, this process never truly stops. Individual galaxies disappear, but new ones continue to enter this strange evolutionary stage. In other words, these “abnormal” galaxies are actually a natural and permanent part of the cosmic cycle.
How Do We Find Ghost Galaxies?
Finding these cosmic ghosts is extremely difficult. They are very faint, spread out over large areas, and often too dim for traditional sky surveys.
Astronomers look for several clues.
One of the strongest signs is the presence of stellar tidal streams—faint ribbons of stars with surface brightness fainter than about 29 mag arcsec⁻².
Another clue is location. Most candidate ghost or stripped galaxies are found within about 150 kiloparsecs of a massive galaxy with a stellar mass greater than 10¹⁰ solar masses.
A particularly interesting signature is an unusually large number of globular clusters. Strong tidal interactions can trigger the formation or survival of these dense star clusters. A dwarf galaxy with far more globular clusters than expected may have experienced a violent tidal history.
Future observatories such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will transform this search. Its deep, wide-field observations will allow astronomers to detect extremely faint galaxies and finally test whether the ghost galaxies predicted by the COCO and NewHorizon simulations truly exist.
Do Ghost Galaxies Challenge Modern Cosmology?
At first glance, these galaxies may seem like a problem for our understanding of the universe.
In reality, they are exactly the opposite.
Current research suggests that ghost galaxies and stripped dwarf galaxies are natural consequences of the standard model of cosmology. They emerge naturally from gravity, dark matter, and the chaotic process of galaxy formation.
“The creation of these galaxies is therefore a natural by-product of galaxy evolution and their existence is not in tension with the standard paradigm.” — Jackson et al.
Rather than breaking the theory, these galaxies strengthen it.
The Ghosts Around Us
The discovery of ghost galaxies forces us to rethink what we consider “normal” galaxy evolution. A galaxy’s life is rarely peaceful. It is a story of failed births, tidal theft, violent mergers, and the endless recycling of cosmic matter.
As we continue to explore the darkness between the stars, an intriguing question remains.
If nearly 30% of dwarf galaxies have experienced unusual evolutionary paths, how many ghostly remnants are hiding around our own Milky Way?
Perhaps our cosmic neighborhood is already filled with invisible galaxies—silent relics of ancient interactions that never managed to shine. They have been drifting through the darkness for billions of years, waiting for telescopes powerful enough to finally reveal them.
With the arrival of LSST and the next generation of observatories, those ghosts may soon step out of the dark.
References
- https://benasque.org/2025udg/
- https://arxiv.org/pdf/2605.11070
- https://www.sciencedaily.com/releases/2026/01/260109220500.htm
- https://uhra.herts.ac.uk/id/eprint/8854/2/stab093.pdf
- https://www.jpl.nasa.gov/news/nasa-reveals-new-details-about-dark-matters-influence-on-universe/
- https://www.quantamagazine.org/the-enduring-mystery-of-the-dragonfly-44-galaxy-20221107/
