Why -
does the particles of matter are very small ?
does the particles of matter have spaces between them ?
does the particles are in continuous motion and are not stable ?
does the particles of matter attract each other ?
These are a few fundamental properties of matter.
Particles of matter are in a state of continuos motion due to thermal and kinetic energy possessed by these particles.
Particles have spaces between them which allows them to disperse and bring about diffusion.