"Dark matter" is the way that lazy scientists sometimes refer to "stuff we can't see".
The problem is that we can calculate how fast the galaxy is spinning, and we can -very roughly! - figure out how much mass is detectable in the galaxy. The problem is that there isn't enough mass to hold the galaxy together spinning at the speed it is. So, either our calculations about the speed the galaxy is spinning is wrong, or there is a WHOLE LOT more mass in the galaxy; mass that we CAN'T SEE. "Dark" matter.
We don't know what dark matter is, or if it really exists, or if there is some other fundamental physical principle that we haven't figured out as well as we think.