Clusters of Galaxies

Perhaps it was at this time, about two billion years after the creation, that clusters of galaxies began to form. A cluster is a group of tens, hundreds or thousands of galaxies held together by gravity. They typically measure tens of millions of light-years across.

The Virgo Cluster is the nearest one to Earth

Clusters are sometimes grouped together into superclusters, which measure hundreds of millions of light-years across. Between the superclusters is almost empty space.

Size of Cluster of Galaxies

Let us use the PennySystem model to try to get a feeling for the size of a cluster of galaxies. A typical cluster is about 3 million light-years across, which if the Solar System were the size of a penny, would be about 80 km (50 miles) across.