The term "world riddle" or "world-riddle" has been associated, for over 100 years, with Friedrich Nietzsche (who mentioned Welträthsel in several of his writings)
and with the biologist-philosopher Ernst Haeckel, who, as a professor of zoology at the University of Jena, "Biography of Ernst Heinrich Haeckel, 1834–1919" (article),
Missouri Association for Creation, Inc., based on 1911 Britannica, webpage:
http://www.gennet.org/facts/haeckel.html Gennet-Haeckel: life, career & beliefs.
wrote the book Die Welträthsel in 1895–1899, in modern spelling Die Welträtsel (German "The World-riddles"), with the English version published under the title The Riddle of the Universe, 1901.
The term "world riddle" concerns the nature of the universe and the meaning of life.
The question and answer of the World Riddle has also been examined as an inspiration or allegorical meaning within some musical compositions, such as the unresolved harmonic progression at the end of "Also sprach Zarathustra" (1896) by composer Richard Strauss, made famous in the film .
"Colorado Symphony Orchestra - Richard Strauss (1864–1949):
Also Sprach Zarathustra" (program notes),
Charley Samson, Colorado Symphony Orchestra, 2004, webpage:
"Classic Records Catalog / LSC-1806: Liner Notes" (description),
Chicago Symphony Orchestra, R. D. Darrell,
Radio Corporation of America (RCA), 1960, webpage: