Dark Matter: How do we know it's there?

This week is all about dark matter - the stuff that physicists have concluded fills interstellar space that doesn't interact with light (hence the reason we can't see it). The search is on for what types of particles this matter might be made of.

But every time a new search is launched, the question arises: How do we know that dark matter is there, to begin with?

Starts with a Bang offers five reasons why physicists are certain that dark matter exists:

1. Galaxies tend to group together in clusters, and observed cluster don't have enough mass to explain this clustering.
2. Galaxies tend to spin like a top, and the rotational velocities of the stars don't match up with what you'd expect if the only mass present was the mass you could see.
3. Dark matter in the early universe left an imprint of oscillations on the Cosmic Microwave Background.
4. Galaxies can collide with each other, and the resulting distribution of stars can't be explained if the visible mass is all that's present.
5. The large-scale structure of galactic clusters has imprints of dark matter in the early universe.

Dark matter, the article concludes, offers an explanation for all these observations, while alternatives (such as modifying our understanding of gravity) cannot explain more than one.