JdeBPIn case anyone wants to pursue the research, the 1945 #ArthurKoestler book is The Yogi And The Commissar, and Koestler makes a one-word malapropism in an astronomical metaphor describing how little Western Europe and North America hear about the U.S.S.R.'s East.
This is then carried into the title of Helena Sikorska's 1947 book about Poland and the Soviet Union, T.S. Eliot explaining the connection in xyr introduction to that book. And contemporary articles that then mention the book include Eliot's quote of Koestler.
Undoubtedly, the error was helped to spread by the advent of the Space Race.
This is the earliest that I've found of 'dark side' mis-used for the far side rather than for the night-time side (as found consistently in many examples before then, including the 1771 Britannica's entry for Astronomy). So Koestler seems to be the origin, in print at any rate.