Scientists Hunt Dark Matter ‘Stars’ That Mimic Black Holes

In 2019, an unusual phenomenon was detected in deep space. Known as GW190521, this event generated gravitational waves – unseen ripples in the cosmos – that were detected on Earth. Initially, these waves were thought to signal the collision and merger of two massive black holes, each with masses several times that of the Sun.

However, another possibility exists. Physicist Carlos Herdeiro from the University of Aveiro in Portugal is spearheading an EU-funded research project called NewFunFiCO to explore alternative explanations.

The NewFunFiCO team comprises physicists and astrophysicists from Spain, Portugal, Italy, Germany, Mexico, Brazil, and China. Their aim is to investigate whether some gravitational-wave signals might originate from exotic space objects that, while theorized, have not yet been directly observed.

“The borderline mysteries of the universe are fascinating,” stated Herdeiro. “Gravitational waves provide a novel way to explore them.”

Listening to ripples in space-time

The team utilizes actual data from the LIGO-Virgo-KAGRA network – a global array of ultra-sensitive detectors in the US, Italy, and Japan that can measure minuscule distortions in space-time. According to Einstein’s theory, space and time form a unified fabric that influences our perception of events.

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These exotic objects relate to some of the deep mysteries in physics such as dark matter.

Since the first detection of gravitational waves in 2015, scientists have identified over 150 merging black hole pairs. However, the theory suggests there could be other exotic entities hidden within the data.

The latest observation run, known as O4, occurred from May 2023 to November 2025 and identified around 250 candidate events, many still under review.

“We are still meticulously examining the data,” said Nico Sanchis-Gual from the University of Valencia, a project co-lead. “There might be signals that don’t entirely match our expectations for black holes.”

The boson star theory

One of the most fascinating candidates being explored are boson stars – hypothetical ultra-compact entities that might resemble black holes from a distance. However, they lack an event horizon, the boundary of a black hole that nothing can escape. From afar, they might appear fuzzy, but internally they would be filled with dark matter particles.

Boson stars might be comprised of ultralight dark matter, potentially involving invisible subatomic particles called axions, vastly lighter than electrons.

Investigating an object the size of a planet, with a mass similar to the Sun but with a vastly different interior, captivates researchers. “It’s mind-blowing,” said Sanchis-Gual.

If they exist, they might occasionally collide and merge, similar to known star-sized black holes, creating detectable gravitational waves. The NewFunFiCO team aims to find the expected signal from such an event in LIGO’s data.

GW190521 may represent such an event. “Should two of them collide, they would produce a gravitational wave signal that better aligns with that signal than two black holes would,” said Herdeiro.

Clues to dark matter

The researchers aren’t only searching for boson stars. They’re also exploring mixed stars, where a neutron star – the dense remnant after a star explodes – has a dark matter core, and also gravastars, exotic objects that mimic black holes but lack the same internal structure and event horizon.

“The aim is to capitalize on the golden era of gravitational wave observations and search for objects never seen before but theoretically possible,” noted Herdeiro