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Any tiny variations in the density of energy in the Macrocosmos was blown up, by the process of inflation, into large scale variations in density in the Universe. As a result, the gas of hydrogen and helium atoms in the young Universe was more dense in some places than others.

About 2 billion years after the big bang the force of gravity made the gas break up into streams and webs of clouds, billions of them. Each cloud of gas shrank under gravity and large empty spaces opened up between them. These clouds have become clusters of galaxies today. They measure tens of millions of light years across.

Clusters are sometimes grouped together into superclusters. They measure hundreds of millions of light years across. Between the superclusters is almost empty space. The galaxies are arranged in sheets and chains within the Universe. Each cluster probably arose from a dense cloud of gas in the big bang.

Galaxies formed within the clusters.

The nearest galaxy cluster to the Milky Way lies in the constellation Virgo.

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