Question

what is the reason of existence of gravity

Answer 1

gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass.

Answer 2

Gravity is the most fundamental energy required for existence of matter.Without gravity not a single particle would exist indepedently. Gravity have bounded the whole universe. Somewhere far in universe gravity contradicts the fact of rectilinear propagation of light by distorting its path..