Galaxies can come in various shapes and sizes, and one of the most unique galaxy types is known as an "ultra-diffuse galaxy." These galaxies are incredibly large, often spanning tens or even hundreds of thousands of light-years across, but they have an unusually low concentration of stars. This means that even though they are enormous, they contain relatively few stars, giving them a ghostly appearance. They are called "ultra-diffuse" because their low stellar density makes them difficult to detect and study.