It’s only 20 light-years wide. It has just 60 stars. And it weighs the equivalent of about 16 suns. So—galaxy or glorified cluster?

Astronomers are buzzing over a cosmic oddball discovered in the constellation Ursa Major that may be the smallest and most dark matter-dominated galaxy ever found. Or, depending on who you ask, it might just be an extremely old star cluster.

“These are fully functional galaxies, but they’re about a millionth of the size of the Milky Way,” Michigan professor Eric Bell said in a statement to Space. “It’s like having a perfectly functional human being that’s the size of a grain of rice.”

This Weird Little Object Might Be the Tiniest Galaxy Ever Discovered

The object, dubbed UMa3/U1, straddles the line between categories. If it’s a galaxy, it should be called Ursa Major III. If it’s a star cluster, its technical name is UNIONS 1—named after the Ultraviolet Near Infrared Optical Northern Survey that found it. What makes it so hard to classify? The problem isn’t its size. It’s what it’s made of.

In the world of astronomy, galaxies are typically massive, dark matter-dominated systems like the Milky Way or Andromeda. Star clusters, on the other hand, are loose, compact groups of stars with little to no dark matter—picture Pleiades. But when you get down to ultra-faint dwarf galaxies (UFDs), the distinctions blur.

“These objects challenge our entire understanding of what defines a galaxy,” researchers from the University of Victoria wrote in a new study analyzing UMa3/U1.

UMa3/U1 is weirdly light for its size. Pleiades is about the same diameter but contains over 1,000 stars. The real question is whether UMa3/U1 is held together by dark matter—like a galaxy—or if it’s simply a tightly bound group of stars that’s managed to survive for 11 billion years.

To figure that out, astronomers ran a few tests. First, they looked at how fast the stars are moving and how long the cluster could stay together. Their model suggests UMa3/U1 is stable enough to survive another 2–3 billion years, a strong case for it being a star cluster.

Next, they examined how the mass is distributed. In galaxies, stars tend to be denser near the center. In clusters, it’s more even. UMa3/U1 matches a cluster—but the faintest, most massive remnants at its core (think white dwarfs or neutron stars) are too dim to detect with today’s instruments.

So, for now, UMa3/U1 remains in limbo. Maybe it’s a galaxy, maybe it’s a cluster, possibly the smallest and oldest of either category. Future telescopes like the Vera Rubin Observatory may help tip the scales.

Until then, it’s a tiny puzzle floating at the edge of the Milky Way—quietly redefining what it means to be a galaxy.