Let's suppose that negative mass exists and that it responds by being repulsed by other matter rather than attracted. So in our galaxy there is a lot more regular mass but there is also some negative mass. Again, a big supposition (and yes I realise that even if there had been, it would have been flung out into deep space long ago). Would the total gravitational effect of the galaxy be based on the positive mass minus the negative mass, and therefore the galaxy would still have an overall attractive quality, or if the negative mass were arranged in such a way as to block the gravitational 'waves' could it actually repulse other galaxies.

 

So say a star is surrounded by a dyson sphere of negative mass, does that star have the gravitational effect minus the negative mass, or does the negative mass cancel it out completely?

 

 

If the dyson sphere were the same mass as the star (but negative) it would exactly cancel, so the combined object would have no gravitational attraction.

Okay. So the total mass is all that matters. Good to know.