Astronomers detect the first globular cluster star stream outside the Milky Way

A new way of measuring a galaxy is judged first on whether it reproduces what other methods already found. "Our results are consistent with previous studies and what they have shown about dark matter in this ultra-diffuse galaxy. We are measuring it with a completely new tool, demonstrating that this method also works beyond our own galaxy," Julie Kiel Holm said in the University of Copenhagen release dated 23 September 2026 ^[6][17]. The release does not report the mass value, its uncertainty, or which telescope supplied the images ^[18]. Agreement is informative in proportion to the precision of the two numbers being compared, and two loose estimates overlap easily.

The physical picture is straightforward. A globular cluster orbiting inside a larger galaxy has stars pulled off it by the host's gravity, a process called tidal stripping, and the escaped stars trail out into a long, narrow line ^[7]. Because gravity sets that line's shape and motion, the stream responds to the mass it is moving through whether or not that mass emits light ^[8]. It constrains how much mass there is and where it sits, not what the mass is made of, and the release says dark matter accounts for roughly 80 to 85 percent of all matter in the universe with its fundamental nature still unknown ^[9]. The remaining 15 to 20 percent is ordinary matter ^[10].

Both objects in this detection are dim. The stream's own signal is weak, and its host galaxy gives off very little light ^[4]. Until recently that faintness made streams very hard to detect and analyse at all, and the release credits larger astronomical datasets and better analysis methods for the change ^[16].

The work centres on one galaxy ^[15]. "We show that a well-established tool from studies of the Milky Way can be used to understand other galaxies, where measuring the distribution of dark matter has traditionally been very challenging," Holm said ^[12]. Sarah Pearson, who contributed to the discovery while employed at the Niels Bohr Institute ^[14], said: "This opens entirely new possibilities. Not only can we now search for globular cluster stellar streams in other galaxies, but in the long term, we may also be able to measure the dark matter content of more ultra-diffuse galaxies" ^[13].