I would have to say that the force of gravity could not have an opposite becuase gravity is created by matter bending space and time, and for there to be reverse gravity there would have to be a negative amount of matter. It is widely accepted by scientists that the closer you get to the centre of the universe the more densly concertrated amounts of matter there is. It is theorised that at a certain point in time the energy from the big bang will stop forcing objects furthur into space and that if the gravity in the centre of the universe will start to pull objects nearby in and as the join the original matter it will become more massive and therefor have more gravity and pull in matter farther away until all matter of the unvierse will be under such pressure that it will change into pure energy and will be the size of the nucleus of an atom. It is also predicted that energy will then turn into electrons and protons and this will force a rapid expansion of matter and space. The big bang. This cycle could go one forever