A Reason Why Astronomers Think There Should be some Dark Matter

Dark Matter is one of those terms we hear a lot about in the news. It is the mysterious, hypothetical 'matter' that helps hold galaxies together. Originally, it was proposed to exist to help explain how galaxies rotate, when the observable amount of 'normal' matter is not sufficient to cause the observed rotation. This video explains a similar idea with another method, which looks at the bullet cluster.