The following is a quote from Wikipedia on the subject of "tests of general relativity".
"Despite Einstein's proposal of three classical tests, the theory was without strong experimental support until a program of precision tests was started in 1959. This program has systematically tested general relativity in weak gravitational fields and severely limited possible deviations from the theory. Since 1974, Hulse and Taylor have studied stronger gravitational fields in binary pulsars. In these regimes, on typical solar system scales, general relativity has been extremely well tested. On the largest scales, such as galactic and cosmological scales, general relativity has not yet been subject to precision tests. Some have interpreted dark matter and dark energy as a failure of Einstein's theory at large distances, small accelerations, or small curvatures."
There is no denying that there have been general sentiments that (currently known) general relativity has not been fully successful in predicting cosmological events at large scale and small accelerations.
This is one of the major reasons we have the theory of MOND and the dark matter hypothesis (which turns out to be correct) to explain the flat rotation curves. If dipole gravity in general relativity had been discovered earlier, we wouldn't be struggling with these problems unless it became absolutely clear that dipole gravity could not explain any of those cosmological problems. It simply reflects the level of confidence physicists have in general relativity today.
If there is a reason that the dark matter could not be made of any other exotic matters, it would be because it is difficult to explain why such an exotic matter should exist there but not here around us if the universe is homogeneous and isotropic.
Dipole gravity is a totally new form of an additional long range gravitational force which is comparable to Newtonian gravity and other three forces, ie, Electromagnetism, Weak and Strong forces, of nature.
One of the most exciting features of dipole gravity is that it shows us the totally new kind of propulsion method to travel the universe. In a sense, it gives matters in the universe the freedom to move around not only in the radial or circular direction around the stars but also in any possible directions in such a way that, for instance, the motion of the bullet clusters can be possible without contradiction.
Dipole gravity is not specifically designed or created to explain the jet phenomenon or the dark matter problem. It came out of the dissatisfaction of the known theory of gravity while there are so many unexplained mysteries in the universe. It is rather a reclamation of the forgotten or, one may say, mistakenly abandoned physics in general relativity.
In lay man’s term, dipole gravity, if verified by experiment, means we are on the verge of the next Scientific Revolution.