On the Origin of Inertia
The original problem was the flat galactic velocity curves and Dark Matter.
Any solution that used time to correct the velocity curves would also show up in other places. Supernova remnants make great pictures so I would see them a lot during my searches. Then I found information about 1987a brightening. I ran the numbers and it was close. The brightening was a collision but could it also be another example of time speeding up.
So it was galactic velocity curves suggesting something like faster time, then the SNR 1987a brightening.
One could not exist without the other.
They are similar, but they are NOT the same.
One, the SNR brightening, represents time moving faster for a small mass as it enters gravitationally weak space.
The other, are large stars orbiting galaxies with high velocities, inconsistent with the square of the distance law.
On the Origin of Inertia,
Screaming Monkey Speculation
Inertia is a property of matter, it is not a property of space.
A small mass can travel with incredibly high velocity in empty space.
A star might also be able to do this.
Inertial mass is relative to the presence of other mass.
Two large stars orbiting close to each other will show the correct amount of inertia and velocity.
Two large stars orbiting very far apart where gravity is >6.674E-11 will not show the correct velocities.
Inertia has gone down. The stars are in normal time, but the weak gravitational field has reduced their inertia.
Space does not give mass the properties of inertia, other mass does.
I have read that the Einstein Field equations work well for large masses and strong gravity. But the equations are more difficult to solve, (or down right squirrelly), for weak gravity.
I think that Einstein may have brushed closed to what I am suggesting. but it would have opened up another can of worms. He was often competing against others for a solution, and this may have looked foolhardy.