[gsm (OP)]

We assume that gravity within a galaxy has a uniform inverse square relation to radius because we assume an idealistic uniform 3-D universe. Within a 3-D universe the orbital velocity should have a inverse relationship to radius, but that's not what is observed. So astrophysicists came up with dark matter to explain this. But why does the universe have to be an idealistic uniform 3-D? Why not 3-D close to a star and 2-D further out. A 2-D universe would yield gravity with an inverse relationship to radius and an orbital velocity that doesn't change with radius - just like what is observed.   

[Farsight]

Because the universe is 3D, not 2D. Sorry gsm. But there other ways to account for flat galactic rotation curves without  dark matter such as WIMPs. One method is inhomogeneous space. If you take a look at Einstein's 1920 Leyden Address, you can see him describing a gravitational field in such terms:

"This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that 'empty space' in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν)...".

Ever heard of galaxies in the expanding universe being described like raisins in a cake? The cake expands, but the raisins don't. The space between the galaxies expands, but the space within a galaxy doesn't. Hence the galaxy is surrounded by a shell of inhomogeneous space. And what's inhomogeneous space? See above. It's a gravitational field! This supplements the gravitational field caused by the matter in the galaxy, and can be expected to result in gravitational anomalies.

Mind you, that's not to say there is no dark matter at all. Neutrinos are dark matter, but they're "hot" dark matter. Relic neutrinos are "lukewarm". And get this: space has its "vacuum energy", and this energy has a mass-equivalence, so you could say that space itself is the dark matter. It's cold and it's dark, it's all around us, and there's a heck of a lot of it.