FAST Just Found Two “Ghost” Clouds in Space That Have Almost No Stars

Astronomers have discovered two unusual clouds of hydrogen near the famous Whirlpool Galaxy that appear to contain almost no stars. The objects, detected using China’s powerful Five-hundred-meter Aperture Spherical Telescope (FAST), could provide an important test of how galaxies form and how some dark-matter structures remain almost completely invisible.

The two objects, called Cloud N and Cloud S, contain roughly 3 million times the mass of the Sun in hydrogen each. Yet astronomers have found no obvious stars associated with either cloud at optical wavelengths.

Their discovery, described in a paper published in Astronomy & Astrophysics on August 4, 2026, may represent a rare example of a predicted type of object known as a reionization-limited H I cloud, or RELHIC.

How Can a Huge Cloud Have Almost No Stars?

At first, the idea seems strange. Stars form when clouds of gas become sufficiently cold and dense for gravity to pull the material together. Over time, these collapsing regions can produce stars and eventually galaxies.

But the universe contains enormous amounts of dark matter, and not every dark-matter structure necessarily becomes a normal galaxy.

According to the standard Lambda Cold Dark Matter (ΛCDM) model, small dark-matter halos formed early in cosmic history. These structures could later merge and grow into larger halos containing galaxies.

However, some smaller halos may never gather the conditions required to produce stars.

Their gravitational pull may simply be too weak to compress their gas. At the same time, the gas can be heated by the universe's background ultraviolet radiation. This radiation comes from energetic sources such as quasars and stars spread throughout the cosmos.

If the gas remains too warm and diffuse, it cannot collapse efficiently.

The result could be a dark-matter halo containing neutral hydrogen—but almost nothing that would make it visible in ordinary optical images.

These hypothetical objects are sometimes described as “starless” or “dark” clouds.

The RELHIC Idea

The researchers, led by Qingze Chen of the National Astronomical Observatories of the Chinese Academy of Sciences, investigated whether the newly discovered clouds could fit this theoretical picture.

A RELHIC is essentially a small dark-matter halo that managed to retain some gas after a major event called cosmic reionization, but whose gas never became cold and dense enough to form stars.

Cosmic reionization occurred billions of years ago when the universe's first luminous sources produced enough radiation to ionize much of the surrounding hydrogen.

For a surviving hydrogen cloud to qualify as a RELHIC candidate, researchers expect several characteristics.

It should be relatively compact and approximately spherical. Its hydrogen should produce narrow radio spectral lines, corresponding to relatively low gas velocities of around 20 kilometers per second. Most importantly, it should lack a detectable stellar component.

The ΛCDM model predicts that there could be a very large population of low-mass dark-matter halos.

Yet astronomers have identified only a small number of objects that might fit this description.

That makes Cloud N and Cloud S particularly interesting.

FAST Searches for Invisible Hydrogen

The discovery was made using the Five-hundred-meter Aperture Spherical Telescope, better known as FAST.

Located in China, FAST is the world's largest filled-aperture single-dish radio telescope. Instead of looking only for visible light, radio telescopes can detect signals produced by material that is difficult or impossible to see with optical telescopes.

One especially important signal comes from neutral hydrogen, or H I.

Neutral hydrogen emits radio waves at a wavelength of 21 centimeters. By detecting this signal, astronomers can map enormous quantities of hydrogen even when there are few or no stars nearby.

The researchers used observations from FAST's FEASTS survey and examined 55 galaxies in the survey sample.

The search produced only two objects that satisfied the required conditions.

Those were Cloud N and Cloud S.

Two Massive Clouds With Almost Nothing to See

The two clouds have remarkably similar properties.

Each contains approximately 3 million solar masses of hydrogen. Their estimated total dark-matter halo mass is around 3.7 billion solar masses.

That means the clouds could contain a surprisingly large amount of hydrogen compared with their overall halo mass.

Importantly, this relationship between gas mass and total mass is broadly consistent with theoretical expectations for RELHIC systems.

The researchers therefore concluded that both objects are compatible with the RELHIC framework.

But there is an important complication.

Both clouds are located approximately 70–90 kiloparsecs from the center of the Whirlpool Galaxy, also known as M51.

That distance corresponds to roughly 228,000–293,000 light-years.

And M51 is not an isolated galaxy.

Could They Be Pieces of a Galactic Collision?

The Whirlpool Galaxy is famous for its dramatic interaction with a smaller companion galaxy. Such interactions can stretch and pull enormous quantities of gas away from galaxies.

This process can create tidal debris—long streams or clouds of material produced by gravitational interactions between galaxies.

That creates a major question about Cloud N and Cloud S.

Are they genuinely small dark-matter halos that never formed stars?

Or are they simply pieces of hydrogen pulled away from galaxies during gravitational interactions?

At the moment, astronomers cannot completely eliminate the second possibility.

The researchers themselves acknowledge that the location of the clouds near the interacting M51 system makes tidal debris an important alternative explanation.

This distinction matters because the two possibilities would tell astronomers very different things.

If the clouds are tidal debris, they would be remnants of a galactic interaction.

If they are genuine RELHICs, however, they could represent examples of the long-predicted population of low-mass dark-matter halos that contain gas but failed to become galaxies.

A Rare Window Into Dark Matter

One of the most fascinating aspects of the discovery is that the objects themselves may not be completely invisible.

Dark matter does not emit or reflect light in the normal way, but if a dark-matter halo contains neutral hydrogen, astronomers can potentially detect the gas surrounding it.

That gives researchers an indirect way to investigate structures that would otherwise be extremely difficult to find.

Cloud N and Cloud S therefore offer a potentially valuable laboratory for testing galaxy-formation theories.

Their existence could help answer a basic question: Why do some dark-matter halos become galaxies while others remain almost completely starless?

However, astronomers are not yet declaring the mystery solved.

The next step is to obtain higher-resolution radio observations. Such observations could reveal the detailed structure, motion and distribution of the hydrogen.

Researchers may then be able to determine whether the clouds have the characteristics expected from independent dark-matter halos or whether their properties instead point toward material torn from galaxies.

For now, FAST has uncovered two strange objects that sit somewhere between a galaxy and an invisible dark-matter structure.

They contain millions of Suns' worth of hydrogen, yet almost no stars.

If future observations confirm that they really are RELHICs, these two “ghost clouds” could become an important piece of evidence for understanding how the universe builds—or sometimes fails to build—galaxies. 

Reference: Qingze Chen et al, A RELHIC twin candidate near the galaxy M51, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202661166