According to the big bang theory, matter clumped into stars because of

The answer is:

According to the Big Bang Theory, matter clumped into stars because of the force of the gravity.

The explanation:

The model of this theory show how the universe expanded from a very high density and very high temperature state and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements.  

After the initial expansion, the universe cooled  subatomic particles and later simple atoms are formed .

Giant clouds of these primordial elements later coalesced through gravity in halos of dark matter, Then the stars and galaxies visible today  formed .

it doesn't. it was imagined up in the middle ages.

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