Another nail in the coffin of Dark Matter and in favour of MOND and similar explanations here.
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Dark Energy is a different topic, and is a placeholder name for whatever causes galaxies to all be moving away from each other, unless they are fairly close and gravitational attraction over-rides it (by several orders of magnitude).
The clever scientists don't really know why this is happening, and there are different ways of calculating the rate of expansion which give different results (although they all seem to be within +/- 10% of each other).
I have spent a couple of weeks reading and trying to understand the page on Ozone Depletion Theory titled What is Electromagnetic Radiation?. I still only vaguely understand most of it, but this sentence grabbed my attention:
Light illuminates matter, but light itself is not visible, it is dark, until it interacts with matter. Given that Earth receives less than 5 x 10-8 % of Sun’s radiation, there must be a lot of dark energy in space that changes in time...
Woo hoo! That might be it. As a matter of fact, there is such a thing as solar sails, as popularised by Arthur C Clark, which use 'photon pressure' to gently but firmly accelerate a spacecraft. The rate of acceleration is painfully slow, but it is constant and cumulative, so they can get up to fair old speeds after a few years or a few decades.
So the logic is this: only a teeny-tiny amount of light emitted by a star hits solid matter in its own solar system or even its host galaxy; most of the light goes out into intergalactic space. A teeny-tiny amount of starlight leaving each galaxy hits solid matter in its own galaxy cluster, and so on and so forth. As star light is nearly as old as the universe, those billions and billions of years worth of starlight from each galaxy cluster is pushing each other galaxy cluster gently but firmly away from itself and the cumulative effect is (just about) measurable (if you call ≈70 km/second per megaparsec distance measurable). By definition the rate of expansion must be gently but firmly increasing.
The (observable) universe doesn't really have a centre, so this leads to its gradual expansion!
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This might be complete nonsense, but it's an entertaining thought at least. And seeing as its the only explanation I've ever seen, I'm going to stick with it for now until a better one comes along.