How sure do we know that there exists dark matter?

This going to be about the Second year Astronomy class Modern Research.
We've decided that the question we would focus our research on will be:
"How sure do we know that there is a dark matter problem?"
The main page that links our group to eachother is Stefans page . So click there to find each subquestion that we have decided on.

The subquestion that I have been dealt is:
* How is the luminous mass in a galaxy determined?

Douwe.

P.S. You can mail me at D.S.de.Wilde@cpedu.rug.nl