Ultra-Diffuse Galaxies

Ultra-Diffuse Galaxies, also known as Invisible Galaxies or UDGs, are incredibly sparsely populated galaxies. They can be as large as the Milky Way, yet only have around 1% of the stars. Due to a lack of gas, they are full of old stellar populations.

Although no formal studies have been conducted on this subject, we can speculate that habitability would be difficult at best, due to the population type (low metallicity) of the stars. This is just speculation, however. Any observer in a UDG would likely see a relatively empty night sky.

Worldbuilding in Practice
"Karnak is an Ultra-Diffuse Galaxy in the Thebes cluster, connected to the Milky Way via an ancient intergalactic gate. It is 25 kpc wide, yet has only around 860 million stars. Although it has little to recommend it in many regards, it is very useful as a stepping stone from the Milky Way to the Thebes cluster."