I am not a scientist, and here is my latest theory of gravity:
Gravitational waves emanating from every atom sometimes add to each other, and sometimes subtract from each other -- just as all types of waves do. The result is that the attractive force of gravity sometimes increases and sometimes decreases.
Inside each galaxy, the tendency is for gravity to increase; outside of each galaxy, the tendency is for gravity to decrease.
There is no dark matter holding galaxies together; there is no dark energy forcing each galaxy away from its neighbors.
If the above is true, how does each atom keep pumping out gravitational energy? Well, all atoms take in electromagnetic radiation from the stars. Possibly, each atom converts some small part of that "positive" electromagnetic energy into the "negative" energy of gravitational waves. Analogies don't always work, and this one might not, but I think of waves coming into a beach, and undertow pulling one swimmer out -- they call that "rip current"; but fifty yards to the north, a return current pulls another swimmer in.