Astronomers have identified what appears to be a massive cloud of dark matter roughly 3,000 light years from our solar system. Using pulsar observations, a team led by Sukanya Chakrabarti detected gravitational effects suggesting an object 60 million times the sun's mass. This could be the first such sub-halo found in the Milky Way.
Galaxies like the Milky Way are thought to be surrounded by halos of dark matter, with smaller sub-halos distributed throughout. These invisible structures are challenging to detect because dark matter neither emits nor interacts with light, only influencing ordinary matter through gravity.
To find one, Sukanya Chakrabarti at the University of Alabama in Huntsville and her colleagues turned to pulsars—rapidly spinning neutron stars that act as precise cosmic clocks. By observing changes in the orbits of pulsar pairs, they measured accelerations caused by nearby massive objects. About 3,000 light years away, they spotted unusual pulls on a pulsar pair and surrounding individual pulsars.
"There’s one pair of pulsars and the [individual] pulsars around it – there’s something in this part of the sky that’s pulling all of these pulsars in this weird direction that we didn’t expect," said team member Philip Chang at the University of Wisconsin-Milwaukee.
Analysis revealed the object spans several hundred light years and has a mass equivalent to 60 million suns. Maps of visible stars, gas, and other baryonic matter show no corresponding features, pointing to dark matter as the likely culprit. However, the researchers note that confirmation is not yet definitive.
This potential sub-halo might be unique in our galactic neighborhood. "There might only be one or two locally, but it depends on the model of dark matter," commented Alice Quillen at the University of Rochester.
Chakrabarti's team aims to map more sub-halos galaxy-wide to probe dark matter's nature. "Our goal is to map out as many of these sub-halos as we can across the galaxy, and we’ve just started being able to do that. Then the ultimate goal is to understand the nature of dark matter," she said. Pulsar binaries are scarce, with only 27 observed precisely enough for such measurements, limiting detections so far. Efforts are underway to use more abundant tracers for broader mapping.