I seem to remember someone here saying that the farther away a galaxy is from us , the mre red shift that galaxy has and thus it is acclelerating . The energy density is driving force , and supposedly it is dark energy which causes this acceleration .
I did some reading up and another explanation for this percieved acceleration of farther away galaxies is that as we look at distant galaxies we are looking back in time , and thus going by the original expansion theory they would have been going faster then that they are now , thus the percieved acceleration of expansion . If this model is correct , then all matter will eventually slow to a halt and initiate the compression and after that and many eons of tense waiting and fretting over the end of the universe you will get the big crunch , and supposedly it all starts again and you get your gravitational singularity.
But the dark energy and accelerating expansion is the latest model going off the latest evidence .