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Scientists Discover Way to Send Information into Black Holes Without Using Energy

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Hubble Uncovers a Ghost Galaxy Made Almost Entirely of Dark Matter

The universe is full of dazzling sights—spiral galaxies glowing with billions of stars, colorful nebulae, and violent explosions of supernovae. But not everything in space shines brightly. Some galaxies are so faint that they are almost invisible, hiding in plain sight among brighter cosmic structures. Recently, astronomers using the Hubble Space Telescope have identified one such extraordinary object: a galaxy that may be 99% dark matter.

This mysterious object, named CDG-2, is one of the most extreme examples ever found of a dark-matter-dominated galaxy. Its discovery and analysis were reported in the The Astrophysical Journal Letters, offering new clues about how galaxies form, evolve, and sometimes nearly disappear.

What Makes a Galaxy Almost Invisible?

Most galaxies are easy to spot because their stars emit light. Even distant galaxies usually appear as glowing smudges in powerful telescopes. CDG-2, however, belongs to a rare class known as low-surface-brightness galaxies. These galaxies contain very few stars spread thinly over a large area, making their light extremely faint.

In simple terms, CDG-2 is like a city with only a handful of dim streetlights spread across many kilometers. From far away, it barely glows at all.

The main reason for this faintness is dark matter—a mysterious substance that does not emit, reflect, or absorb light. Scientists cannot see dark matter directly, but they know it exists because of the way it influences gravity. In most galaxies, dark matter outweighs normal matter, but in CDG-2, this imbalance appears to be extreme.

Finding the Unfindable: A Clever New Method

Detecting such a ghostly galaxy is incredibly difficult. Traditional galaxy searches rely on light from stars, which CDG-2 barely has. To overcome this challenge, astronomer David Li and his team at the University of Toronto used a clever indirect approach.

Instead of searching for stars, they searched for globular clusters.

Globular clusters are tight, spherical groups of tens of thousands of stars, bound together by gravity. They usually orbit normal galaxies like satellites. Importantly, globular clusters are dense and bright compared to the faint galaxies that host them. This makes them easier to detect, even when the parent galaxy itself is nearly invisible.

By using advanced statistical techniques, Li’s team scanned astronomical data for small, tight groupings of globular clusters with no obvious galaxy around them. This method led them to identify ten known faint galaxies and two new dark galaxy candidates—including CDG-2.

Confirming CDG-2 with Three Powerful Observatories

To confirm that CDG-2 was truly a galaxy and not a coincidence, astronomers used data from three major observatories:

  • Hubble Space Telescope

  • Euclid space observatory

  • Subaru Telescope

Hubble’s sharp vision revealed four globular clusters located very close together inside the Perseus galaxy cluster, about 300 million light-years from Earth. This tight grouping strongly suggested that something massive—like a galaxy—was holding them together.

When astronomers combined observations from Hubble, Euclid, and Subaru, they detected a faint, diffuse glow surrounding the clusters. This glow was the long-sought evidence of a hidden galaxy.

“This is the first galaxy detected solely through its globular cluster population,” Li explained. Under conservative assumptions, these four clusters likely represent all of CDG-2’s globular clusters.

A Galaxy with Almost No Normal Matter

Once CDG-2 was confirmed, scientists analyzed its properties. The results were astonishing.

  • The galaxy’s total light output is equivalent to only 6 million Sun-like stars.

  • About 16% of this visible light comes from just four globular clusters.

  • An estimated 99% of its total mass is dark matter.

For comparison, the Milky Way contains hundreds of billions of stars. CDG-2 is tiny, faint, and nearly empty—yet still massive because of dark matter.

So where did its normal matter go?

Stripped by a Crowded Cosmic Neighborhood

CDG-2 lives inside the Perseus galaxy cluster, a crowded environment packed with massive galaxies and intense gravitational forces. Astronomers believe that as CDG-2 moved through this cluster, gravitational interactions stripped away much of its hydrogen gas—the raw material needed to form new stars.

Without gas, star formation shut down. Over time, the galaxy was left with only a few old stars and its tough, tightly bound globular clusters. Dark matter, which does not interact easily, remained behind and now dominates the galaxy’s mass.

Why Globular Clusters Matter So Much

Globular clusters play a crucial role in discoveries like this. Because they are extremely dense and strongly held together by gravity, they can survive violent cosmic events that destroy other structures.

This makes them reliable tracers of hidden galaxies. Even when a galaxy loses most of its stars and gas, its globular clusters can remain as signposts pointing to an invisible host.

The Future: Finding More Dark Galaxies

CDG-2 is likely not alone. Astronomers expect many more dark or nearly dark galaxies to exist, especially in dense clusters. The challenge is finding them.

Upcoming and ongoing missions are making this easier:

  • Nancy Grace Roman Space Telescope

  • Vera C. Rubin Observatory

  • Euclid space observatory

These projects will generate enormous amounts of data, mapping billions of objects across the sky. To handle this flood of information, astronomers are increasingly using machine learning and advanced statistics to spot unusual patterns—like lonely clusters of globular stars with no visible galaxy.

Why CDG-2 Matters

CDG-2 is more than a curiosity. It challenges existing ideas about galaxy formation and highlights the powerful role of dark matter in shaping the universe. By studying such extreme objects, scientists can better understand:

  • How galaxies lose their normal matter

  • How dark matter behaves in different environments

  • Why some galaxies shine brightly while others fade into near invisibility

In the end, CDG-2 reminds us that the universe is not just made of what we can see. Hidden in the darkness are countless structures quietly shaped by forces we are only beginning to understand.

Reference: Dayi (David) 大一 Li 李 et al, Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adddab

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