There's also a recent theory, that suggests, that the space expanding is accelerating due to a very distant merger of gigantic (on the scale of mass of the entire visible universe) black holes, caused by long gravitational wave pressure (as I understood it), emitted by that merger. The theory is interesting for it drops the need for cosmological constant.

https://don-beaver.livejournal.com/196412.html

Mix of Russian explanation + paper in English published in MNRAS (closer to the end of post).

The theory's main premise is that "gravitational waves don't have gravitational mass", and spewed quite a lot of debate.

I wish I'd know physics better to check their math.