What force causes the material in a nebula to contract?

The primary force that causes the material in a nebula to contract is gravity. As gas and dust particles in the nebula come closer together, their gravitational attraction increases, drawing more material into the center. This process can lead to the formation of stars as the contracting material heats up and undergoes nuclear fusion. Additionally, other factors like pressure and turbulence can influence the contraction process, but gravity is the dominant force.