I knew a guy that built the best gravity meters in the world. He was hired by the military to try to measure “anti-gravity” as a solution to why the missiles weren’t landing where they were supposed to. This was years ago, and perhaps before the idea of dark matter was invented.
He couldn’t measure any anti-gravity (too difficult in actual on-site field experiments) - but he didn’t discount the idea. Although I guess that dark matter is the opposite of anti-gravity. Although the idea of anti-gravity may account for the variations that they observe?
All of the other natural forces (magnetic, electro-magnetic, etc.) have positive and negative attractions, so why wouldn’t gravity?
Dr. Robert Forward of Hughes Aircraft?