While studying the Coma galaxy cluster in 1933, astronomer Fritz Zwicky uncovered a problem. The mass of all the stars in the cluster added up to only a few percent of the heft needed to keep member galaxies from escaping the cluster's gravitational grip. He predicted that the "missing mass," now known as dark matter, was the glue that was holding the cluster together.

Dark matter, as its name implies, is matter that cannot be seen. It does not emit, absorb, or reflect light, nor does it interact with any known particles. The presence of these elusive particles is only known through their gravitational pull on visible matter in space. This mysterious substance is the invisible scaffolding of our universe forming long filamentary structures—the cosmic web—along which galaxies form.

Even more confounding is that dark matter makes up the vast bulk of the universe's overall mass content. The stuff that stars, planets, and humans are made of accounts for just a few percent of the universe's contents.

Astronomers have been chasing this ghostly substance for decades but still don't have many answers. They have devised ingenious methods to infer dark matter's presence by tracing the signs of its gravitational effects.

One technique involves measuring how dark matter's gravity in a massive galaxy cluster magnifies and warps light from a distant background galaxy. This phenomenon, called gravitational lensing, produces smeared images of remote galaxies and occasionally multiple copies of a single image.

A recent study of 11 hefty galaxy clusters found that some small-scale clumps of dark matter are so concentrated that the lensing effects they produce are 10 times stronger than expected. These concentrations are associated with individual cluster galaxies.

Researchers using the Hubble Space Telescope and the European Southern Observatory's Very Large Telescope in Chile discovered with unprecedented detail smaller-scale distorted images of remote galaxies nested like Matryoshka dolls within the larger-scale lens distortions in each cluster's core, where the most massive galaxies reside.

This unexpected discovery means there is a discrepancy between these observations and theoretical models of how dark matter should be distributed in galaxy clusters. It could signal a gap in astronomers' current understanding of the nature of dark matter.

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