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/

Unexplained acoustic or radio frequency signals linked to space or the upper atmosphere that scientists still have not fully pinned down.
 
1. The Wow! signal (1977)
A strong, 72‑second narrowband radio signal at the hydrogen line (around 1420 MHz) detected by the Big Ear telescope in Ohio, famously annotated “Wow!” on the printout. No repeat has ever been reliably observed from the same sky region, and proposed explanations now include a powerful magnetar flare hitting a neutral hydrogen cloud, but this remains unproven and controversial.
 
2. Fast Radio Bursts (FRBs)
Millisecond‑long flashes of radio waves, often from well beyond the Milky Way, with huge, implied energies but still uncertain progenitors. Models range from magnetar flares and collapsing neutron stars to more exotic ideas; multiple sub‑classes (repeaters vs non‑repeaters, different frequency drifts) suggest more than one underlying mechanism.
 
3. Mysterious infrasound in the stratosphere
Solar‑powered balloons carrying infrasound microphones at about 20–22 km altitude have recorded low‑frequency rumbling signals a few times per hour whose sources cannot yet be identified. Known contributors (thunder, waves, human activity, rockets) do not explain the repeating, unexplained component, which may involve unknown atmospheric dynamics or distant surface sources.
 
4. The Upsweep (NOAA, Pacific Ocean)
A persistent, upsweeping low‑frequency sound discovered by NOAA hydrophones in the central Pacific in 1991, strongest in spring and autumn each year, origin undetermined. Hypotheses include undersea volcanic or hydrothermal activity, but no specific geological source has been definitively matched to the signal.
 
5. The “Whistle” / chorus emissions near Earth
Spacecraft and probes routinely record eerie, whistling audio when plasma‑wave data from Earth’s magnetosphere are shifted into the audible range. The general mechanism (electromagnetic plasma waves interacting with charged particles) is understood, but detailed source regions and wave‑particle processes behind some specific spectral chorus structures remain active research topics.
 
6. The Space Roar (ARCADE excess)
The ARCADE balloon experiment measuring faint radio emission found a uniform radio background about six times brighter than predicted from known galaxies and early‑universe sources. No consensus explanation exists; possibilities include numerous faint radio galaxies, exotic early‑universe processes, or unknown astrophysical populations, but all struggle to match the amplitude of the observed roar.
 
7. Other unexplained NOAA ocean sounds
From NOAA’s catalogue, several named underwater sounds remain not fully explained, despite being localized approximately in the ocean.
 
Bloop: A powerful, low‑frequency sound detected in 1997; now often associated with large icequakes, but without a confirmed, directly imaged source.
 
Train, Whistles, and a few others: Have plausible links to seafloor seismic or cryogenic activity, yet individual events and their precise mechanisms are still not firmly tied to specific physical sources.
 
These examples span true deep‑space radio transients, near‑Earth plasma wave audio, a puzzling radio background, and enigmatic infrasound and hydroacoustic signals at the interface between Earth, ocean, and space, each still missing a fully agreed upon physical explanation.
 
References
Turner, B. & LiveScience. (2023, May 17). Bizarre, Unexplained Rumblings in Earth’s Atmosphere Puzzle Scientists. Scientific American.
 
(2026, January 22). Wow! Signal. In Wikipedia, https://en.wikipedia.org/wiki/Wow!_signal
 
(2026, February 18). List of Unexplained Sounds. In Wikipedia, https://en.wikipedia.org/wiki/List_of_unexplained_sounds
 
(2025, January 6). The Wow! Signal: A Lingering Mystery or a Natural Phenomenon? SETI Institute. https://www.seti.org/news/the-wow-signal-a-lingering-mystery-or-a-natural-phenomenon/
 
(2015, May 8). On the Origins of Fast Radio Bursts and Perytons: A Statement. CAASTRO. http://caastro.org/project/on-the-origins-of-fast-radio-bursts-and-perytons-a-statement/