I see gravity as a diff Version of Wind in an atmosphere, trying to achieve the ultimate equilibrium, only diff its on a stupendus huge scale so the universe from our perspective looks flat
Galaxies graviate towhard oneanother, so to keep in perfect harmony the univers keep expanding. It looks like the expansion is going faster and faster but the Space itself expand at a fixed rate since the total mass inside it stay the same.
This is just my teory and it just as any other teory you prefer out there.