Gravity is the primary force that attracts particles within a nebula. Here's why:

* Nebulae are vast clouds of gas and dust. These particles are spread out, but they still have mass.

* Gravity is a universal force that attracts any two objects with mass. The more mass an object has, the stronger its gravitational pull.

* While the gravitational force between individual particles in a nebula is weak, the combined gravitational pull of all the particles in the cloud can be significant.

* Over time, gravity causes the particles in a nebula to clump together. As they get closer, their gravitational pull increases, further attracting more particles.

* This process of gravitational collapse is how stars are born. The dense core of a collapsing nebula becomes hot and dense enough to initiate nuclear fusion, marking the birth of a star.

So, gravity is the driving force behind the formation of stars from nebulae.