During accretion, gravity doesn't generate enough energy to fuse 2 dust particles together. So how does dust become pebbles and pebbles become boulders?
Accretion is the process of forming larger bodies from smaller ones. [1] During accretion, gravity does not generate enough energy to fuse two dust particles together. [1] Instead, large dust grains accrete to form small pebbles, small pebbles accrete to form large pebbles, and pebbles accrete to form rocks. [1, 3, 4] Once the process is started, it can actually speed up, as the pebbles grow into larger and larger rocks. [2] Scientists believe that the planets appear from rotating gas disks surrounding newly formed stars, known as proto-planetary discs. [3, 4] Pebble-sized objects in these discs then clump together to form cores of would-be planets. [4]
During the process of accretion, gravity does not generate enough energy to fuse two dust particles together. But as particles became larger, they exert a larger gravitational force and attract smaller particles to them [1].  Accretion can occur as new bits of material join something or glom onto it. In the case of the solar system, it was a swirling disk of gas and pebbles that clumped together to form a family of planets [2].  As the rocks in the disk slow down and heat up, they fly through the gas near a larger rock. It’s a phenomenon similar to how water in a pond slows a sinking rock. These flying pebbles spiral down to land on the surface of larger rocks nearby [2]. Pebble by pebble, a giant planet is born.