no one has answerew this yet...
Well I will then.
1. Go to wikipedia
2. Go to the library. (if you are going to discuss something with a college professor get a book. Trust me there are tons of them.)
3. Listen to me (but dont expect everything I say to be right)
4. Ask a teacher.
Dark matter is supposed to be the force that holds galaxies together. It is an invisible matrix that provides the excess mass needed to account for all the galaxies today. Supposedly the universe does not have enough mass to produce all the galaxies it has also galaxies have a stronger gravitation than there suppose to.
This is where dark matter gets involved. Currently we are searching for a candidate for dark matter, but the evidence is all there. The best candidate is something called an axion. This is a mysterious particle that fixes many problems in quantumchromodynamics. (I dont know what that is so dont ask me)
Goodluck, and get a book!