It is about complementarity in all fields. This complementarity concerns natural phenomena and the phenomena of the cosmos. With its two black and white parts, the Yin & Yang is the perfect representation, not only of complementarity, but also of duality within the same entity.
The Yin nature, black, represents the night, the darkness and the moon. As for the Yang, it represents perfectly the light, the day, the sun. The complementarity and duality between Yin and Yang proves that the forces that govern our universe are not only complementary, but above all indistinguishable, like your couple !