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 .

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.