Author of "My Big Toe" (MBT) trilogy Physicist Thomas Campbell explains what happens during the Double Slit Experiment and why it happens.
"Physics has no idea of the fundamental principles that require particles to exist as probability distributions before measurement collapses the wave function to a physical particle (measured physical state). They just know that it is so. MBT derives from fundamental first principles why particles must always exist first as probability distributions. Thus MBT derives Quantum mechanics from first principles. This solution completely resolves the apparent contradiction of wave particle duality generated by the double slit experiment. MBT further shows that the same concepts (reality must always exist first as probability distributions) apply just as much to the macro world as the micro world -- it is a general principle -- not a special principle only valid at subatomic scales."- Thomas Campbell