That is a fascinating experiment and whilst sciwntists sorta knows what's going on, it is just theory but it explains a lot of nature.
Scientists describe the 2 slit experiment as the photons having superposition. That is a probability matrix describing where the particle is. Once you measure the photon position is resolved, by probability, to having passed through one of either of the slits.
The bit scientists struggle with is the interference wave pattern that results if you fire one photon at a time. The current thinking is that whilst in superposition the probabilities interfere with each other giving the wave interference pattern. If you measure every photon, ie it is not in superposition, the wave pattern disappeares. This also describes how photosynthesis works, scientists think.