Without yin there is no yang and without yang there is no yin. Think of it as a chicken and egg question. The fundamental belief behind the Yin Yang theory is that everything in the universe can be divided into yin or yang. The catch is that nothing is entirely yin or entirely yang. Each has a kernel of the other at its core. This makes balance possible in the universe. For example day (yang) changes into night (yin) and then back to day again in an endless cycle. Day is dependent on night for its existence and night is also dependant on day for its existence. Therefore we can say that they are complementary and opposite.