When the mass density decreases in the voids between large scale structures sp does the strength of gravity in those locales. Recall gravity falls off in strength at 1/r^2.

 A matter only universe is called the Einstein Desitter universe.

The wiki link is extremely poor for this particular toy model 

https://www.google.com/url?sa=t&source=web&rct=j&url=https://en.m.wikipedia.org/wiki/Einstein%E2%80%93de_Sitter_universe&ved=2ahUKEwimzKjByfjaAhXMzVMKHZMTBCUQFjACegQIBhAB&usg=AOvVaw0_ZbKqQmM_j5zuKIkZYw6s

(on phone atm)

but the essential detail is that due to matter distribution into LSS, the average mass density reduces as matter starts to pool up into galaxies etc. So the average density decreases thus so does the average global gravitational potential.

Edited May 9, 20187 yr by Mordred

9 hours ago, Mordred said:

So the average density decreases thus so does the average global gravitational potential.

Cheers.