1)Learn about the search for dark matter. 2) Determine the ratio of visible matt

1)Learn about the search for dark matter.
2) Determine the ratio of visible matter to dark matter in a galactic cluster from real observation data.
Additionnal:
-for the second point: please use simple formulas so i can understand.
-add a fee references please.
-Please it is very important that you clearly add the exact references (specify the page if necessary).