[derleth]

Dust absorbs light, blocking it (creating dark spots) and getting hot (creating light spots in a different frequency range). We know what interstellar dust looks like. Dark matter doesn't look like that; it doesn't look like anything. We only know about it because its gravity affects the things we can see.

NASA has a brief page on dark matter and something else we know little about, dark energy:

http://science.nasa.gov/astrophysics/focus-areas/what-is-dar...