1) missing matter
2) gravity gets weaker by distance (beyond the inverse square law,
naturally)

It seems slightly obvious that a third conjecture would be something
like:

3) missing distance

In other words, the distance from the center of the galaxy to the tip
of the spiral arm is far greater than what is measured.

We know there is a black hole at the center of the galaxy.

We also assume that black holes look like a big black hole, with all
sorts of light coming from around the edges.

But we don't see that in the center of the galaxy, do we? I know, I
know, there is alot of light covering it up, in the way, right?

Maybe our picture of black holes is wrong, and the surface area of the
event horizon is essentially equal to zero. Instead of a black hole
with light around it, like a nice big halo, maybe a black hole
including the event horizon really is a single point, and the light
coming from around the edges of a zero-dimensional point, obviously now
just looks like light.

That would mean... in addition to the black hole "hiding" mass in the
center of the galaxy, it is also "hiding" distance?

It seems like something worth at least worth refuting anyways.

In any case, it is a testable hypothesis. If we directly measure the
surface area of the event horizon, we should affirm General
Relativity's prediction, or affirm the zero prediction and explain the
spiral galaxies at the same time?