Why do bullet clusters provide evidence for the existence of dark matter?

okay , Pretend you had two toy cars with a attached tub full of marbles on the top of it , and you drove them straight into each other , well , the two cars (galaxies) would instantly come to a stop but the marbles (dark matter ) would pass straight through and keep on going and fly all over the place

The collision of two clusters of almost equal mass results in most of the dark matter being separated from the galaxies. Its independent gravitational effect then becomes apparent through a comparison of the X-ray and optical images.