Surely if mass and energy are essentially related, then it is perfectly logical to deduce that negative mass would be related to negative energy? And once you get back to energy then force fields due to plasma movements cannot be too far removed either?
This suggests that we have to try to understand (positive) mass and energy far better than we do now before we can advance meaningfully into the concepts of the corresponding negatives.

How little we know. I would like to order a vehicle that uses dark fluid to repel gravity and creates thrust please.

I propose that gravity reverses and becomes repulsive at approximately 1.5 million light years. It becomes more and more strongly repulsive, reaches a peak, and then decreases trailing off to zero.
This does away both with dark matter and also dark energy. It would explain why most galaxies are accelerating away from each other – leaving no need for cosmological expansion or dark energy.
It also explains gravitational rotational rates without the need of dark matter. Galaxies are pushing dust and gas into the interstitial space between galaxies. This dust and gas means that each galaxy or small galaxy cluster is surrounded by a womb of material at a distance that repulsive gravity operates. This repulsive womb, along with the pressure from other galaxies, holds outer stars in place, explaining higher than expected rotation. You may read the justification for this theory here, along with responses to objections at the bottom: https://www.reddit.com/r/MyTheoryIs/comments/87pcgq/what_dark_matter_is/ I think that General Relativity can be adjusted such that we keep time dilation, BUT ditch curved or dilated space. I.e. we should work with flat, 3D , Euclidean space + time dilation. I explain about this in the notes at the bottom of the article.

So has Einstein suddenly become passe? 'Positive' masses do not attract each other. Rather, they are attracted 'to' each other via their deformation of space-time geometry caused by their respective mass. 'Negative' mass implies negative space-time which, as a closed-curve geometry, obviates the possibility of us ever directly observing such a phenomenon.
'Dark energy' isn't an energy at all. A more correct reference would be to interpret the observation of increasing rate of expansion over time as us (i.e., the present) accelerating away from more distant objects (distant past), not the distant objects accelerating away from us. The increasing rate of expansion is a consequence of the (thermodynamic law of) conservation of space-time. As matter is converted into energy, space-time expands accordingly. The ratio of space-time to matter is huge, thus the apparent magnitude of expansion observed.

Gravitational lensing and, now, gravitational waves are in common use and require spacetime curvature.

The aether anybody??

The recently proposed theory(s) regarding the possibility that the speed of light is not really a constant, but possibly has changed over the eons time, has the potential to account for all the gaps that have been filled in by the "darks". I, for one, certainly don't think its out of the realm of possibility that the speed of light has not been precisely the same from the very beginning... but this notion seems to throw many into a tizzy of sacrilege!

So, hypothetical matter, hypothetical energy, and a hypothetical fluid! All this to explain unseen forces affecting the movement of the matter in the universe..... If you saw a steel washer, sliding around on the table, would you inspect the hole in the middle for its motive force? Unlikely. The average person would look under the table for a large magnet being moved around. Another more believable, perhaps, and logical hypothesis for the anomalies observed in the motions of the matter in the universe could be;... Astronomical observations have shown that black holes had already formed 800 million years after the 'big bang'. Those black holes would, after some 12 billion + years be on the periphery of our universe, would have absorbed all their accretion disks, and would therefore be completely invisible. Also, they would, by now, exist in their trillions upon trillions. So our universe would be encased in a 'shell' of these trillions upon trillions of black holes. Their combined gravitational forces would have noticeable effects upon all the matter within this shell. In addition, all the matter would be attracted by this shell, hence the accelerating expansion of our universe. Eventually all the matter of the universe would become black holes, and as they would all attract one another, the universe would then start contracting, finally ending in the hypothetical, 'big crunch.' This 'crunch' would be of such violence, that enormous energies would be released, resulting in another 'Big Bang.' ('And so on ad infinitum')

This ground breaking book is set to be presented at the 2019 Association of College and Research Libraries (ACRL) conference in Cleveland, OH, from April 10th to 13th, 2019.

@NIK, yes!!!!!!!!!!!!!
That is what I have been thinking for some time. The problem with the big bang theory is EVERYTHING DOES NOT COME FROM NOTHING. The big bang theory is the 2nd half of a theory missing the first half.
The black hole.
The black hole is the front door and the big bang is the back door OF THE SAME THING. So we do have a pulsing universe.