An object moving through space at a constant velocity has no acceleration. Therefore no force is acting upon it. To change its velocity, there must be an acceleration. To get that acceleration, a force must act upon the moving object.
The velocity at which the universe is expanding is increasing. That means there is an increase in acceleration. That increase can only come about due to a force.
Thank you. That's probably the most clear and concise explanation I've seen on the subject. You must be a good teacher, 'cause I don't feel so dumb as reading the rest of these posts. Wasn't it Lord Kelvin who said physics should be clear enough to be explained to a barmaid, or some such thing?
Great quote. Whoever said, I think he was right.