Which force causes particles in outer space to join together into larger and larger objects?

Air resistance, or drag, is a force that opposes the motion of objects through air. In outer space, there is no air or atmosphere, making air resistance non-existent. Therefore, it cannot play any role in the formation of larger objects in space, as this force only applies in environments where air is present.

Tension is a force transmitted through a string, rope, or other flexible connectors when they are pulled tight by forces acting from opposite ends. This force is not applicable in the context of space where particles are primarily influenced by gravitational forces rather than by tensions in physical materials, as there are no solid or liquid connectors in the vacuum of space to experience tension.

Friction is the resistance that one surface or object encounters when moving over another. In the vacuum of space, there are no surfaces to create friction, making this force irrelevant to the process of particle aggregation. Thus, friction does not contribute to the joining of particles into larger objects in outer space.