Black Holes Group Travelling Across the Milky Way

Astronomers have recently identified a swarm of more than 100 stellar‑mass black holes moving together through the Milky Way inside the dissolving globular cluster Palomar 5. The Gaia mission’s precise mapping of stars revealed that the star cluster Palomar 5 has long, delicate tidal streams stretching tens of thousands of light‑years around the Milky Way, and models only fit the data if the cluster’s core contains over 100 black holes. These are stellar‑mass black holes (roughly tens of solar masses each), the remnants of massive stars that formed when the cluster itself was born billions of years ago. As Palomar 5 orbits the Milky Way, tidal forces from our Galaxy pull stars away, creating extended stellar streams that trace its path through the halo, while the heavier black holes sink toward and dominate the cluster’s centre.
 
Simulations show that in about a billion years the visible cluster will fully dissolve into a stream of stars, and nearly all that remains bound will be a compact group of black holes orbiting the Galactic center together – effectively a migrating black‑hole swarm embedded in the Milky Way halo.
 
The Palomar 5 result implies that other diffuse globular clusters and stellar streams may also be the debris of clusters whose cores are rich in black holes, so such swarms could be common throughout the Milky Way halo. Dense pockets of stellar‑mass black holes in clusters are excellent environments for black‑hole mergers that produce gravitational waves detectable by instruments like LIGO and Virgo, so systems like Palomar 5 may be important factories of the events we observe.
 
Beyond this clustered swarm, models suggest there may be on the order of 100 million isolated stellar‑mass black holes roaming the Milky Way, some of which have been tentatively measured via microlensing, indicating individual objects racing through the Galaxy at around 160,000 km/h. On much larger scales, astronomers have also reported a runaway supermassive black hole about 20 million solar masses speeding through intergalactic space with a long trail of newly formed stars, showing that even giant black holes can be ejected and travel independently of any galaxy.
 
References
Ralls, E. (2026). This Star Cluster Hides an Anthill of More Than 100 Black Holes Traveling Through the Milky Way. Earth.Com.
 
Starr, M. (2026, February 20). Gaia Detected an Entire Swarm of Black Holes Traveling Through the Milky Way. Science Alert.
 
(2022, June 10). Hubble Determines Mass of Isolated Black Hole Roaming Our Milky Way Galaxy. NASA. https://science.nasa.gov/missions/hubble/hubble-determines-mass-of-isolated-black-hole-roaming-our-milky-way-galaxy/