No, gravity and dark energy are not related. In fact, dark energy seems to be the 'opposite' of gravity. Simply put, dark energy acts to push matter away from each other, while gravity acts to pull matter together. read more
Dark matter has gravity. When they looked at the way galaxies rotate, the distribution of matter they could see, did not match the orbital dynamics. The simplest theory is they couldn't see it all. There is now more evidence, but this shows the link. read more
In fact, there's a know particle that does just that: the neutrino; and it was a candidate for the dark matter constituent for a while, until further calculations showed that it didn't have the right mass and other characteristics to produce the correct distribution that was observed. read more
There are no known modifications to Einstein's gravity that can reproduce these predictions, even after-the-fact, without also adding dark matter. Zosia Rostomian An illustration of clustering patterns due to Baryon Acoustic Oscillations, where the likelihood of finding a galaxy at a certain distance from any other galaxy is governed by the relationship between dark matter and normal matter. read more