This has been placed in philosophy, it is a metaphysical idea.
Metaphysics is a generic pointer which could assist in the development of new physics and science, I have spent many year studying cosmology from the life cycle of stars, to the evolution of planetary systems. I have a clear conceptual understanding of relativity, time dilation and other matters.
This idea may have already been debunked, but it is in my opinion it is important to publish it because it might develop a new line of thinking.
The idea is simple and I hope I can explain it clearly in the next few paragraphs.
Gravity is the enigma, which could be considered the final puzzle of modern physics, not to say that this would be the end, but maybe the start of the next generation of physics, but until gravity can be integrated in the quantum realm, it seems we can't develop a theory of everything.
We currently turn to mass and dark matter to explain the gravitational effect in the universe, my thought was very simple mass may not have anything to do with it, it might be purely down to space, space is responsible for the gravitational effect rather than matter. We might need to stop looking at matter as the source of gravity and turn out attention to space instead.
The universe is expanding at ever accelerating rates as space expands there is less matter to bind it, it could very well be the lack of space where matter resides where gravity is greatest.
That's it! it is only a passing thought, but I feel that there may be some merit in the idea.