From Abracadabra to Zombies
In a nutshell: Yin-yang is an ancient Chinese idea that stands for the two sides of things that complement each other.
Thousands of years ago, the Chinese words yin and yang referred to the shady and sunny sides of a hill or valley. The symbol for yin-yang comes from the yearly pattern of shadows cast by a long pole stuck straight up in the ground.
Ancient Chinese sky gazers found that as the Sun appears to move over the Earth, the shadow of the pole is a little longer or shorter each day, depending on what time of year it is.
Yin and yang are ideas found in Chinese stories written long before the rise of modern science. The ancient Chinese knew nothing about atoms or molecules. Nor did they know that the Earth revolves around the Sun.
The yin-yang symbol is usually thought of along with the ancient Chinese religion called Taoism. Yin and yang stand for the two sides of things, like mountains and valleys, or the inside of a bowl (whose shape is concave) and can't exist without the outside of the bowl (whose shape is convex).
Taoists thought of yin as the basis for things cool, soft, dark, moist, still, passive, and female. Yang was thought of as the basis for things hot, hard, light, dry, active, and male.
According to legend, the Chinese emperor Fu Hsi (2852-2738 BCE) said that the best state for everything in the universe is a state of harmony represented by a balance of yin and yang.
Modern science has found other explanations for such things as hot and cold, light and dark. The temperature of things depends on the movement of their atomic particles. Light and dark depend on the presence or absence of photons. Soft and hard depend on what kinds of molecules a thing is made of.
Also, there is no reason to believe that the pattern of yearly shadows on the planet has any connection to most of the other things that go on in the universe. Still, the yin-yang symbol is beautiful. It reminds me of a spiral galaxy hugging itself.