Well, generally in Taoism, we believe that there are spiritual
forces(ling) everywhere and spirits, both good and bad is all
around.
Taoism believes that it is the Tao, which is the source of the
universe, give rises to all things. First, we have the ultimate
one, the Tao, and only the superme Patriarch Tai shang laojun
exists in the universe. Then the Tao splits into 2, yin and
yang,and with this 2 powerful forces, many things are formed. Tai
shang lao jun forms himself into 2 divines - heavenly lord of the
primival and heavenly lord of the numinous treasure. Finally, the
last stage, where all is done and the universe is formed, tai shang
lao jun reappear as the 3rd divine.
After this, the 3 divines(three pure ones) using their vital breath
forms many other heavenly and earthly divines, such as Jade
Emperor, Three Divine Officials etc.. In the begining there is
neither good nor bad, because of the forming of the cosmic, and the
spliting of yin and yang, comparision, we have the dark side being
formed. I mentioned earlier that there are spiritual forces in the
whole cosmic, thus spirits can be form in the nature and some lean
towards the opposite way of the Tao or the Yin side, then there are
demons etc. Humans who practiced evil arts, magic will fall to the
dark side too.
However, the Yin energy is NOT bad, neither Yang is good. both are
merely energies that pushes the comsic moving.