In a preprint released in August 2025, researchers from the Max Planck Institute for Astrophysics reported a newly identified cloud composed entirely of dark matter. The structure has a mass several million times greater than that of the Sun and extends across hundreds of light-years. Owing to its proximity within the Galaxy, it would appear larger in the sky than either the Sun or Moon if it were visible.
A simulation shows the distribution of dark matter surrounding the Milky Way (yellow). Bright areas represent dark matter clumps, or subhalos, and the brightest ones would contain dwarf satellite galaxies of the Milky Way. Image Credit: Max Planck Institute for Astrophysics.
Dark matter, which is thought to exceed regular matter in the cosmos by more than five-to-one, can only be detected by gravity. Its true nature is mysterious. However, models of the universe’s evolution indicate that the Milky Way is not just surrounded by a diffuse dark matter “halo” weighing in at a trillion times the mass of the Sun, it also contains countless smaller clumps, known as subhalos, that drift through space among the stars.
If verified, the dark matter cloud would be the first subhalo seen in the Milky Way. If astronomers can locate more, their size and dispersion may help constrain the nature of dark matter.
It’s an exciting start of a new era. He’s not sure the evidence for the cloud is strong enough for a firm claim of discovery, but the technique the team used—tracking the decaying motion of celestial clocks called pulsars—holds great promise.
Niayesh Afshordi, Astrophysicist, Perimeter Institute
Afshordi is not sure if the evidence for the cloud is strong enough to make a formal claim of discovery, but the approach the team used (tracking the decaying motion of celestial clocks known as pulsars) shows enormous potential, he added.
“This is the beginning of a new type of astronomy,” Afshordi added. He hopes the new object represents the “tip of the iceberg.”
The current cosmological hypothesis states that dark matter is a massive, cold particle that clumps and affects galaxy formation by attracting ordinary matter. The clumping might explain why astronomers find so many little dwarf galaxies around bigger ones.
This also suggests that massive galaxies should be filled with numerous dark matter subhalos, ranging in mass from billions of Suns down to the mass of Earth. If dark matter were made of brighter, warmer particles, it would be less clumpy, resulting in fewer of these small subhalos.
Improved maps of the Milky Way have encouraged astronomers to search for signs that dark matter subhalos are influencing its structure. One promising approach focuses on star streams: long, thin trails of stars stretched by the Galaxy’s gravity and looping around its disk. Interestingly, some of these streams appear broken, as if something unseen has plowed through them. Still, astronomers have yet to definitively link any of these disruptions to a dark matter subhalo
The Milky Way is kind of a messy place. There are lots of other objects … that could pass through these streams.
Mike Boylan-Kolchin, Astrophysicist, University of Texas, Austin
Instead, Sukanya Chakrabarti, an astrophysicist at the University of Alabama in Huntsville, and her colleagues searched for hints in pulsars, which are the superdense relics of destroyed stars. Pulsars are rapidly spinning neutron stars that emit rhythmic bursts of radio waves, sweeping past Earth like the beam of a lighthouse, sometimes hundreds of times per second.
Her team focused on pulsars that are part of binary systems, paired with either another pulsar or a companion star. As the pulsar orbits its companion, the radio pulses shift in frequency, speeding up as it moves closer to Earth and slowing down as it moves away. This rhythmic variation allows researchers to measure the orbital period with remarkable precision and monitor how it changes over time.
Chakrabarti’s team used pulsar data archives dating back more than a decade to study how the orbital periods of 27 pulsar couples deteriorated with time. Well-understood effects, like the emission of gravitational waves from spinning masses, are expected to gradually shrink the orbits of these systems. But if researchers detect any extra orbital decay beyond what’s predicted, it could point to the gravitational pull of a massive, unseen object nearby acting on the pulsars .
Of the 27 pulsar binaries, a few in the same region of the sky showed similar amounts of abnormal decay. According to the team's simulation, something weighing around 10 million solar masses is tugging them all down. A high concentration of normal matter, such as stars or gas, might be the cause.
However, the researchers discovered nothing when they searched the star catalog of Europe's Gaia spacecraft, the most comprehensive current list, and a comparable map of molecular gas clouds. A big black hole might explain the excess decay, but it would have to exceed the supermassive black hole at the galactic core while remaining undiscovered.
We’ve gone through every possible data set. We don’t know, but we tend to think that it’s more likely to be a subhalo.
Sukanya Chakrabarti, Astrophysicist, Cornell University
Others remain unconvinced.
“It’s going to take a lot more for people to definitively see this as a detection,” said Boylan-Kolchin.
If it holds up, even the first subhalo might reveal information about dark matter. According to Boylan-Kolchin, the team’s modeling implies that the mass in the dark matter cloud is irregularly distributed, indicating an uncommon type of dark matter.
“That’s the exciting prospect, I’d say,” added Boylan-Kolchin.
According to Chakrabarti, their measurements will improve when additional cosmic clocks are discovered and monitored over an extended period of time. Fortunately, the team can build on worldwide efforts utilizing pulsars to search for a background “hum” of gravitational waves from merging supermassive black holes.
“With upcoming data, the precision is going to improve,” concluded Chakrabarti.