Describe how a spiral galaxy is thought to form. drag the items on the left to the appropriate blanks on the right to complete the sentences. (items can be used once or not at all.) the extra density in some areas increased the gravitational pull, and then reversing the expansion that occurred due to the cosmological expansion, forming protogalactic gas clouds. the gas cloud begins to form stars, but the star formation does not use up all the gas. the angular momentum of the original gas cloud causes the gas cloud to accelerating and flatten out as it collapses, gas particles collide, and the gas spread out, thereby forming the disk of a spiral galaxy out of the leftover gas.