Clearly, any matter that pops into existence surrounded by nothing at all is going to feel enormous forces at its boundaries. As this puny pea of matter is sucked large by its surrounding nout the area where matter meets nothing grows. This growing area naturally has a larger sucking-factor and so the 'expansion' of the matter gets faster.
That Einstein bloke really should have thought of this.
Oh! Oh! Wait!
Gravity doesn't suck! It's actually the Earth accelerating as it expands making us stick to the surface.