The calculation has been done by an American physicist, analyzing the radio waves of different galaxies. The electric charge of the photons is less than 10-46e. Although this number is very small, it is higher than previously thought, so the electric charge of the photons may be higher than expected.
Photons are neutral in physics, without charge. Admitting that photons are charged would mean reviewing some of the physics and, for example, adapting what is thought about the development of the universe.