>> we don't really understand gravity

I would say that what we don't understand is how the universe is structurally layered, vibrationally/dimensionally. That 5/6ths of the matter is missing lines up with our physical dimension (of energy and matter) being only one of six onionish dimensional layers, all contained within the same 3-space.

The five dimensions are three of space (x,y,z), one of time (t), and one of dimensional vibration (?). That's the structure, but I have neither the maths nor the details to divine how everything is interrelated. All the dimensions' matter does combine to contribute to the inertia of the galaxies we can see.

What is particularly difficult is that we can only measure our dimension's matter & energy with our tools, which are, of course, made of our matter & energy. In specific high-energy experiments, however, we can get some crossover of, e.g., anti-particles appearing on our side of the "wall".

That's the structure, as far as I understand it. The rest will require much maths and experiments and exploring the unknown. That is what Einstein did to expand Newton's understanding. There's at least one more level-up needed to encompass our measured corner-case phenomena.