You may find this helpful: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve If you look at a 33rpm spinn

You may find this helpful: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve If you look at a 33rpm spinning record, you'll see that any point on the edge of the label is moving a lot slower (linear speed) than the record's edge ... even though it has the same 'angular velocity' (33 rpm). But galaxies are not solid like a record, and as the article explains, "most stars in spiral galaxies orbit at roughly the same speed" (that's linear speed). So the edges 'fall behind.' (That was not recognized until about 1975.)