When a beam of light or electron is shot through a dual slit contraption, a bar-code like interference is created on a screen behind the slits - as waxing and waning intensities of light or electron.
If a detector is placed next to a slit, to detect the light or electron, the bar code interference vanishes and a dot like structures appear - as if the electron or photon has a preference to one of the slits over the other.
This is the Duality of Electron or Light where it is observed to be Particle or Wave. I now would explain why this happens.
When no detector is placed, the Space Geometry is the same - and the slope of Space-Time is the same. But in simple terms, the Ethereal Space density is the same around the two slits. When a detector is placed, the matter of the detector, whether in terms of mass or electro magnetic properties, modifies the Space-Time around the present slit in its vicinity. This causes the Wave to coalesce towards a slit with a bias and appear as a particle.
Because we do not acknowledge Ether -- or the Dark Energy -- in this Experiment, the main stream Science is baffled by this Duality and cannot explain it classically. This is then placed in the same category of bewilderment as some of the other Quantum effects - like Heisenberg Principle, Quantum Spin and so on.