Ba-Wari Observatory
During the nighttime, rural Ba-Wari is completely dark, surrounded by black mountains and with no light to be spoken of. The whole purpose is for an excellent view of the stars and galaxy that surround it so Ba-Matoran astronomers could study and discover.
Dark, cirque valleys like the middle of the Obsidian Mountains are ideal for Ba-Matoran because of the correct amount of pressure from the altitude of said mountains (they also happen to be ideal for wild Cloud Sheep). Despite the name, the Obsidian Mountains are not made of said substance, rather they are named for their blackness.
(also idk why I thought of Ba-Matoran as astronomers, I associated Gravity with space because gravity is sometimes used as a celestial subject and may or may not also be the title of a space movie so yeah)