What Is Interfering With Photons?

"What should we expect to happen when these experiments are performed only one photon at a time?" "We might still expect penumbras. But what we surely could not observe is any place on the screen, such as x, that receives photons when two slits are open, but which goes dark when two more are opened." The interfering entities behave exactly like photons, except they cannot be seen. They travel at the speed of light, bounce off mirrors and refracted by lenses or opaque barriers. This phenomenon would go entirely unnoticed if it were not for this phenomenon in theory.