[quote=@WilsonTurner]
And the edges are...?
[/quote]

The galaxy is approximately 80,000 lightyears in diameter, so the edges would be:

-80, 80
0, -80
-80, -80
-80, 0
80, -80
80, 0
80, 80
0, 80

Which doesn't really fit the circular shape of the galaxy, so the corners should be considered to extend out into dark space.