In reality god particle is known as higgs boson, to understand higgs boson we need to understand mass clearly. In easy words mass is measurement of inertia of a body, let's have a example, a car is in rest and you are trying to push the car and move it forward direction at the same time you push the car you will also experience the mass of the car. Anything which don't have mass like photons that can achieve the speed of light.

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In quantum field theory I have told you that whole universe is filled with fields which we can't see. When we give energy in this fields new particles are formed and when we extract energy from fields particles get destroyed, like that there is one field also which is known as higgs boson, when we give energy in this field higgs boson (god particle) particles are created.

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This higgs boson is the source of mass to all elementary particles how? let's know,

When massless particles enter in higgs field they start interact with higgs field and whichever particle will react more it will gain more mass and which will react less it will gain less mass. Like photons which interact very less with higgs field that's they have very less mass or negligible. 

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This higgs boson theory was given by Peter Higgs and this theory is now proved also by CERN.