Dark Matter/Dark Energy

Dark matter and dark energy are two of the biggest mysteries in science today. Although we don't know what they are, scientists would not have added them to our models of the universe without strong evidence that they represent real phenomena (see the Hallmarks of Science). In a 2-3 paragraph essay, identify one piece of evidence for the existence of either dark matter or dark energy, and describe it in detail. Explain why the observations you have chosen are scientifically convincing evidence for the existence of dark matter or dark energy.