Clusters of galaxies. Let us continue our journey going to larger and larger scales in the Universe. We have already stated that the galaxies are the building blocks of the Universe and so, to make a map of the structure of the Universe on the very largest scale, we need a three-dimensional map of where the galaxies are located. It turns out that the galaxies are far from radomly distributed in space. They generally form rather irregular structures, as we will see in a moment, but there exist some large systems of galaxies, the largest bound system that we know of in the Universe, which are called clusters of galaxies.