If there is antigravity, and i do believ there is, then there should be such things as negative mass, and negative energy, possibly even negative velocities. Some theories ive read about say that anitgravity is the opposing force to gravity. Opposing forces to gravity can include electromagnatism. Why doesn't your hand go through a table when you lay it down? This can be a misconception for this type of antigravity is just an illusion created by electromagnetism and/or other forces.

there is also electrogravitation. If the electromagnetic force can affect gravitation, then since we can control electromagnetism, we can control gravity.

reducing radiant energy densities in a volume of space below absolute zero could produce antigravity, using forces to modify spacetime somehow could bring about antigravity, or compressing spacetime with high quantities of force to produce a spontaneous quantum generation reaction may produce certain quantities of antigravity. Some of these things are relativley impossible, especially at this time, maybe right after the Big Bang, but the conditions are different now (and thank God!). Antigravitation may actually be the cause of spontaneous quantum generation and not the product of it, no one knows. I dont know what this does for you people out there who read this, but it is guiding me to an answer that hasnt yet been reached, but soon may, in my opinion.