The mystery of dark matter remains one of the most profound puzzles in modern astrophysics. We know it outweighs ordinary matter by a factor of five, yet it neither shines nor absorbs light. Its presence is betrayed only by the pull of gravity, shaping galaxies and bending light from more distant objects. For decades, scientists have sought not just to prove dark matter exists, but to understand what it actually is.

A new study ¹ by Tom Broadhurst and collaborators has taken a striking step in this quest, using the James Webb Space Telescope (JWST) to watch how stars flicker behind a massive galaxy cluster known as Abell 370. This work focuses on a spectacularly distorted spiral galaxy that appears as a long arc of light—appropriately nicknamed the “Dragon Arc.” By studying subtle variations in brightness of stars within this arc, the team has found evidence that may help distinguish between different possible forms of dark matter.

Dragon Arc

Source: Tom Broadhurst et al (2025) ApJL doi: 10.3847/2041-8213/ad9aa8

The Dragon Arc as a natural laboratory

The Dragon Arc exists because Abell 370, a huge cluster of galaxies, acts like a gravitational lens. Its immense gravity bends and magnifies light from background galaxies. In certain regions, the bending is so extreme that single stars in distant galaxies can briefly flare into visibility. This effect, called microlensing, occurs when individual stars (or other compact masses) within the cluster further focus the light from behind.

JWST, with its extraordinary sensitivity, has now detected dozens of such microlensed stars along the Dragon Arc. Most of these stars are not blazing blue giants like the famous “Icarus” discovered by Hubble in 2018, but rather evolved red giants, stars near the end of their lives. These stars are faint by cosmic standards, yet lensing boosts their brightness enough for JWST to spot them across billions of light years.

A skewed distribution

The team noticed something curious: the microlensed stars do not appear evenly around the predicted “critical curve” (the region of strongest lensing). Instead, they cluster in a band about 4.5 kiloparsecs wide, shifted slightly—by about 700 parsecs—toward the inside of the curve, closer to the cluster center. This asymmetry is unexpected. If dark matter were smoothly distributed, the band should be narrower and more symmetric. The observed skew hints that small-scale irregularities in the cluster’s dark matter distribution are at play.

Testing dark matter models

Two leading ideas for dark matter predict different kinds of small-scale structure.

Cold Dark Matter (CDM): The standard model envisions dark matter as a collection of heavy, slow-moving particles. It naturally forms countless small clumps, or “subhalos,” within larger halos. These subhalos should create local lensing distortions, broadening the microlensing region. But CDM predicts that the strongest distortions occur outside the critical curve, the opposite of what JWST observes in the Dragon Arc.

Wave (or Fuzzy) Dark Matter: An alternative idea is that dark matter consists of ultralight particles behaving collectively like a quantum wave. This “ψDM” model predicts that interference patterns—like ripples overlapping on a pond—pervade the halos of galaxies and clusters. These fluctuations can shift the critical curves inward, producing exactly the kind of wide, skewed distribution seen in the Dragon Arc.

By matching the data, Broadhurst’s team found that a wave dark matter particle with a mass around 10⁻²² electronvolts—so light that its quantum wavelength spans about 10 parsecs—fits the observations remarkably well. Strikingly, this is the same particle mass inferred from the internal motions of small dwarf galaxies, suggesting a coherent picture across very different scales.

A quantum wave on cosmic scales

This is not just an academic exercise in curve-fitting. The distribution of microlensed stars in the Dragon Arc provides a new, independent test of dark matter theories. For decades, CDM has dominated simulations of cosmic structure, but it struggles with certain small-scale phenomena, such as why dwarf galaxies have large, smooth cores instead of dense peaks. Wave dark matter naturally explains such features and now appears consistent with the lensing evidence from JWST.

Another galaxy, nicknamed the “Jupiter Arc,” shows a similar skew of microlensed stars toward the inside of the critical curve, strengthening the case. If future JWST monitoring confirms this pattern across more systems, it could signal that dark matter is indeed a quantum wave on cosmic scales.

Looking Ahead

The story is far from finished. The team notes uncertainties, such as how variations in stellar populations within the Dragon Arc affect the brightness distribution, or how gas in the cluster may smooth lensing signals. More data—deeper, more frequent JWST observations—are needed to pin down these effects. Still, the results already hint that dark matter may not be just a cold, inert particle, but something stranger and more elegant: a vast cosmic Bose–Einstein condensate rippling across galaxies.

For now, the Dragon Arc offers a rare glimpse into the invisible. By catching the brief twinkle of stars halfway across the universe, we may be uncovering the true nature of the dark matter that surrounds us all.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.

References

Tom Broadhurst et al (2025) Dark Matter Distinguished by Skewed Microlensing in the “Dragon Arc” ApJL doi: 10.3847/2041-8213/ad9aa8 ↩

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