Clusters of Galaxies
Perhaps it was at this time, about two 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 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.
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