What force do scientists think was responsible for clumping matter together to form the first stars after the big bang

Dark matter and gravitational force

Explanation:

Dark matter refers to those invisible matters that occupies more than about 75% of the universe. They are made up of particles that are subatomic in nature. These matters can exert a force of gravitation that has the ability to attract majority of the matters present in the universe.

Stars are often formed by the accumulation of the interstellar gas, and dust particles. They are present in almost all the directions in the universe. They are formed when the huge clouds of gases gets collapsed because of its increasing gravitational force.

Soon after the big bang explosion, the interstellar particles and matters were combined due to the effects of dark matter and after their accumulation, the mass increased and these clouds of matter and gas got collapsed due to the gravitational force. Due to this disintegration, the first stars were formed.  

So, force of gravity and dark matter equally contributed a force that led to the formation of initial stars.