Dark Matter
What we're made of, what everything we can see and touch and feel and smell is made of, is like an impurity in the stuff of the universe, maybe 4%.
The image shows, in xrays in red, the hot gas from two colliding galaxy clusters, with the distinctive bow-shock shape from the collision. In white, we see the visible light from the galaxy clusters. In blue, we see the actual mass distribution, inferrered from gravitational lensing of the background. Read Cosmic Variance and Galactic Interactions for excellent explanations of the observations.