Short answers, pulled from the story.
Albert Einstein proposed that light is made of discrete packets of energy in 1905. This proposal emerged from his attempt to explain the photoelectric effect where light shining on a metal surface ejects electrons only if the light's frequency exceeds a specific threshold. The concept was so controversial that even Max Planck initially rejected the notion that light itself was made of particles.
The word photon was coined by the American physicist and psychologist Leonard T. Troland in 1916 to describe a unit of illumination for the retina. Gilbert N. Lewis popularized the term in a letter to the journal Nature on the 18th of December 1926. Lewis used the word to describe the energy units that Einstein had theorized and his usage was quickly adopted by the physics community.
Current experimental limits place the photon mass at less than 10 to the power of negative 53 grams. This number is so small that if a photon did have mass, its lifetime would exceed 10 to the power of 18 years. Experiments designed to detect these effects have consistently yielded null results reinforcing the standard model's prediction that photons are strictly massless.
Arthur Compton provided definitive proof of wave-particle duality for photons in 1922 through his scattering experiments. He showed that photons carry momentum proportional to their wave number a phenomenon now known as Compton scattering. It took over two decades of experimental verification to convince the physics community that light possessed a dual nature.
Sheldon Glashow Abdus Salam and Steven Weinberg won the 1979 Nobel Prize in physics for developing the electroweak interaction theory. This theory unifies the photon with the W and Z bosons in the Standard Model of particle physics. The photon's role as a gauge boson is a consequence of the symmetry of the electromagnetic field.
The bending of starlight by the sun's gravity was first confirmed during the solar eclipse of 1919. Observations of starlight bending around the sun provided evidence for Einstein's theory of general relativity. This phenomenon known as gravitational lensing shows that photons are affected by gravity as their normally straight trajectories may be bent by warped spacetime.