Unlike magnetism and electricity there is no repulsive force in gravity
in expanding universe. Gravity is mediated through time differential
produced by differential expansion of space around large masses.  
When a mass composed of billions of atoms with electrons moving
with tremendous velocities is placed in the time differential, the inner
motion of the electrons is converted into external linear motion which
is perceived as the attractive force.  Gravity produces acceleration
towards slower expansion of space and slower time which is at the
center of the large masses. Gravity is a weak force as it is mediated
through time differential and not by direct interaction of the particles.  

For gravity to be repulsive the expansion of space will have to be
faster in the center of a celestial body. This cannot happen in an
expanding universe. In a contracting universe however gravity may be
repulsive.  The contraction of space should be fastest in the center of
celestial bodies producing a faster time and reversing the time
differential and producing repulsive gravity thus completing the
missing symmetry in this force.
WHY GRAVITY IS ALWAYS ATTRACTIVE