In 2015, astronomers were examining the Coma galaxy cluster and noticed something odd: huge, almost invisible "fluff balls." These galaxies are as large as our Milky Way but shine hundreds of times fainter. They were dubbed ultra-diffuse — meaning "extremely smeared out." At first, they seemed like rare oddballs, but soon it became clear: such galaxies are abundant in the universe, possibly as numerous as normal ones.

Over the past ten years, scientists have built computer models and conducted observations with the best telescopes. It turned out that ultra-diffuse galaxies don't fit the usual patterns. For instance, typically, the larger the galaxy, the more massive its invisible halo of dark matter. But in these "fluff balls," the relationship is broken: at the same size, the halo can be much lighter or heavier than expected. Moreover, in some of them, stars contain surprisingly many heavy elements, as if these galaxies had a turbulent youth. They also have suspiciously many globular clusters — dense "clumps" of hundreds of thousands of old stars. Ordinary dwarf galaxies don't have such riches.

Why are these galaxies so puffy and dim? There are several ideas. Perhaps they were born in rotating gas clouds that prevented stars from gathering into a dense center. Or they were once normal galaxies but lost their gas due to supernova explosions or tidal forces from neighbors. Or maybe they were "puffed up" by dark matter that behaves differently than we think. So far, there is no single answer.

The review, published for the tenth anniversary of the discovery, takes stock and looks ahead. In the coming years, new telescopes like Euclid and Roman will find thousands of such galaxies. Then, perhaps, we'll figure out what they are: failed giants, a special kind of dwarf, or something entirely new.