The Yin-Yang symbol dates back at least to the 3rd century BCE and is a symbol meaning (among other things) balance in all things. It represents everything that exists and is the perfect balance of dark and light. Tracing the pattern of the sun throughout the year will produce a yinyang symbol on a grid. It is similar to the Golden Spiral, a mathematical construct present in nature and corresponding to the Fibonacci sequence. The ratio is present in everything from DNA molecules to spiral galaxies. (Yeah, it’s about numbers—another hint that science and spirituality are really the same thing.)
Yin represents female or dark energy. All things hidden, mysterious, negatively charged—not submissive but indulgent, reserving power for protection, defense, and the safe nurturing of life in dark recesses.
Yang is masculine manifesting energy. Warm and dynamic, Yang runs where Yin would stay, shouts where Yin would be silent, and burns while Yin freezes. The two kinds of energy complement each other even as they contradict each other. You can’t have one side of a coin or an up without a down. One is not complete alone.
Tiny specks of alternate energy inside each larger form, remind that in light there is always a little darkness and in the darkest time, there is always light. In creation, there is destruction and in even total annihilation, something new is formed. When Yin and Yang energy is balanced, creation is possible. The symbol is a reminder to seek balance in all things and to embrace the contraries.
Other symbols such as the Celtic spiral and the Egyptian ankh are worth looking at, too, if you like this sort of thing.
The Celtic single spiral represents the universe and the triple spiral can mean a number of things—past-present-future, mind-body-spirit, life-death-rebirth. The ankh is about life, death, immortality. Sometimes referred to as the Key to the Nile, it reminds me a bit of a Celtic Cross, as well.