Caelgender is a xenogender that shares quality with outer space or shares the aesthetic of space, stars, nebulae, etc.
Caelgender's original coining post can no longer be found. However, the term appears in a Pastebin document belonging to Cade, one of the mods of the now-defunct original MOGAI term repository, MOGAI-Archive, dated September 27th, 2014. Therefore, the term was most likely created on or before that date, by the Archive mods or one of their followers.
The flag was posted by a later archive site, Pride-Flags-For-Us, on January 30, 2016, but attributed to prolific MOGAI contributor, pastelmemer.