[martincmartin]

The Andromeda Galaxy, presently 2.5 million light-years away, is being pulled toward our galaxy and will eventually collide with it.

To support an inverse square law, we'd need to know the mass of our galaxy, and the mass of the Andromeda galaxy. We have the speed, computed from various blueshift methods. We'd also need to know the rate at which the speed changes.

Do we know any of those three? The mass of the Andromeda galaxy is very different if you assume its filled with dark matter, vs. only has visible matter. Which mass is used to confirm the inverse square law? If it includes dark matter, you have circular reasoning: dark matter is hypothesized to make the inverse square law work within the Andromeda galaxy. If it doesn't include dark matter, then why does the dark matter in the Andromeda galaxy contribute to its rotational motion, but not the attraction to the Milky Way?

[lutusp]

My reply was only in response to this comment --

> Gravity decreases in proportion to distance squared between objects, but its not observed beyond a certain distance.

-- not to the question of what detailed properties gravitation possesses.

[martincmartin]

Ah. I took the comment to say the inverse square isn't observed beyond a certain distance, not that gravity wasn't observed at all beyond a certain distance.