What causes the fundamental forces?
Three of the four fundamental forces are caused by particles.
The electromagnetic force is mediated by the photon. The force can be explained in terms of the exchange of photons.
The weak nuclear force is mediated by the W and Z bosons. Radioactive beta decay converts a neutron into a proton by emitting a
The electro-weak theory says that at high energies the photon and the Z boson are interchangeable and the two forces have been unified.
The strong nuclear force is a residual effect of the colour force which binds quarks together. The gluon is the mediating particle.
Physicists are trying to unify the colour force with the electroweak force.
Gravity is different and in fact is not a force. Gravity is caused by the curvature of spacetime. Mass, energy and momentum cause spacetime to curve which in turn tells matter how to move.