how does the dipole repeller effect work?

no such thing as negative gravity.

It's actually just *less* gravity. Our galaxy and its neighbors are between an area of space with more mass (plenty of galaxies) and another area of space with less mass (fewer galaxies). We are being pulled toward the more mass-filled area because it has more gravity than the emptier area, and if you spend too much time thinking about it you could give yourself the illusion that the empty area is pushing us away.