Which force is the only one known to act on massless particles such as photons?

In Newtonian terms, gravity acts on mass, but in Einstein's general relativity it acts on energy and momentum as well. That is why even light, which has no rest mass, follows curved paths near massive bodies. This was famously confirmed during the solar eclipse observations of 1919, which measured the deflection of starlight by the Sun. The result helped make Einstein's theory internationally famous. Gravity's ability to affect light is one of the clearest signs that it is more than a simple pull between masses.