Astronomers often speak of dark matter and dark energy, terms that are confusing for the public and not very descriptive. In brief, both are invisible substances, meaning that they do not interact with any type of light, from radio to gamma-rays, and their existence is only inferred because of the effect they have on the motion of material that we can see. Here we will look at the evidence for dark matter, as well as the ideas scientist have postulated for what it might be. The dark energy will be the subject of another posting.