Short answers, pulled from the story.
James Clerk Maxwell published a paper describing electricity and magnetism as parts of one electromagnetic field in 1864. This publication marked the first major unification of these two phenomena which scientists had previously treated as unrelated forces.
The Standard Model organizes all known fundamental interactions into four distinct categories including gravity, electromagnetism, the strong interaction, and the weak interaction. Three of these forces operate through the exchange of gauge bosons between particles while gravity remains described by general relativity as spacetime curvature.
Theodor Kaluza extended General Relativity to five dimensions in 1921 to suggest that gravitational curvature in an extra spatial direction behaves as an additional force similar to electromagnetism. Oscar Klein refined this idea in 1926 by proposing that the fourth spatial dimension be curled up into a small unobserved circle within what is now called Kaluza-Klein theory.
Sheldon Glashow, Abdus Salam, and Steven Weinberg received the Nobel Prize in Physics in 1979 for their insights on the electroweak theory which they revised independently in 1967. Experimental confirmation arrived when Carlo Rubbia's team produced the Z and W bosons at CERN in 1983.
Theoretical physicists have not yet formulated a widely accepted theory because trying to merge the graviton with strong and electroweak interactions leads to fundamental mathematical problems. The resulting theory fails to be renormalizable under standard procedures leaving no single equation that currently describes all events in nature.