The oxygen breathers were there first. In other words it started as a undomed colony. Then none breathers got added (probably due to some cargo transfering I was doing). Then all the oxygen breathers got removed, leaving 1 or 2 million none breathers and the planet became a domed colony
While it makes perfect sense that the planet become a domed colony, I don't see how my none breathers could have been there in the first place without a dome. Well at least I get my facilities.