The concept of antiparticles arises from the framework of quantum field theory, which describes particles and their interactions based on quantum mechanics and special relativity. According to this framework, every particle has an associated antiparticle with the same mass but opposite charge.
However, when it comes to particles like photons and gravitons, the situation is different. Photons are particles of light and are their own antiparticles. They have zero rest mass, travel at the speed of light, and have no electric charge. Since they have no charge, there is no concept of "opposite charge" in the case of photons, which means there are no antiphotons.
Similarly, gravitons are hypothetical particles that are postulated to mediate the force of gravity. However, our current understanding of gravity, as described by Einstein's general theory of relativity, does not yet incorporate the principles of quantum mechanics. Therefore, the existence of gravitons as quantized particles is still a topic of ongoing research and theoretical exploration. As of now, there is no confirmed experimental evidence for the existence of gravitons or their antiparticles.
In summary, the absence of antiphotons and antigravitons is a result of the specific properties and characteristics of photons and the current limitations in our understanding of quantum gravity.