The modern science states that the Universe started from a singularity, a single point in space and has been expanding ever since. If we assume that this is true, it may indicate that everything in the Universe is made of the same material, same proto-particles which were there at the very beginning and constitute any substance including elementary particles.
Let’s assume that the Universe is a system of antagonistic forces (easy thing to do if one sees the human world as a projection of the Universe). What would be a force opposing to the force that causes the Universe to expand? Probably a force that tries to take the Universe back to its original state – single point. It empowers all proto-particles dissipated throughout the Universe and forming various shapes and objects to ‘look for their original home base’ in such a way that physical bodies attract each other creating what is called Gravity.