I have been teaching high school Physics for the past two years.  We demonstrated centripetal force by playing crack the whip in small groups in the gym.  They can feel that they would move in a straight line away from the circle if they let go of each others' hands. 

'Centrifugal force' may be the most incorrectly used term in popular media, so what is it?

When most people use the term centrifugal force, they mean centripetal - like in the example of twirling a can on the end of a string.   Centripetal force is not a new and different sort of force from gravity or electricity, centripetal instead means 'toward the center' - so like a can on a string or one of those spinning carnival rides, you are held in place by a center-directed force.

How so?   

For 75 years, some astronomers have believed that the universe contains unseen or ‘dark’ matter that must make up about five-sixths of the matter in the cosmos. The conventional theory of gravitation, based on Newton’s ideas and refined by Einstein 92 years ago, dark matter's existence would help explain the motion of galaxies and clusters of galaxies on the largest scales.