system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists know how much dark matter a galaxy has or doesn't have?
gpt
You take a galaxy and measure the rotation speeds of the stars. When you plot them on a graph of orbital speed Vs their radial distance, you'll see that it doesn't add up. There should be more mass present to account for all that speed. Then you figure out how much mass should be there for the stars to be going that fast and voila. (Simplified version)