What causes gravity?
Max Planck, who introduced his quantum theory in year 1900, used
the electronic circuit model of the atom for the derivation. The
hydrogen atom consists of a single electron and a single proton.
Obviously, gravity is an electric phenomenon. My own investigations
have shown that there is an excellent correlation between the
Coulomb forces between two hydrogen atoms and the force of gravity
between them. For more details you can do an internet search for
the book: "The Secret of Gravity".