The nebular theory states that star systems form by the gravitational collapse of a cloud of gas and dust that, to match observations, must flatten into a disk within which planets can form. Why would this flattening occur?

Gravity made the dust and gas in the nebula collide, flattening these materials into a disk.

Explanation: The nebula theory is a scientific explanation of how the planets and our solar system were formed. It suggests that our solar system was formed more than 4 billion years ago due to the effect of gravity on a cloud of gas and dust which were flattened into a disk. The  gravity was so great that the matter in the nebula were made to collide against each other until they were compacted to form our sun.