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 .
