Is dark matter science or faith?

Science.

With the sole caveat that you use it in the manner in which it is used by astrophysicists and not as it is used by the popular press.

Dark matter is a label put on an anomaly in gravitation for which we have no explanation but many observations.

It is, in a sense, gods way of reminding humans how insignificant they truly are. Our galaxy is not unique, our star is not unique, our planet is not unique, and now it seems that ordinary matter, composed of atoms, is only a minor fraction of the mass of the universe. Science breeds far more humility than religion.