Hmmmmm, I've been thinking (OK, OK, I know you'll say that's a bad idea) that all the talk about Dark Matter, Dark Energy & etc must surely beg the question that if the Dark stuff exists, anyone travelling through space will be encountering it which would surely slow a spacecraft?

An object, when propelled by a force, will continue in the direction of that force until acted upon by another force?

The Voyager 1 spacecraft is now about 11,500,000,000 miles from Earth (Voyager 2 some way behind) and still travelling at over 35,000 miles per hour and it's not being "propelled", the fuel on board is only used to orient the craft so why ain't it slowing down if it's encountering this Dark "stuff"? Just wondering like 😮

The speed was "got up" by it's initial launch speed plus the slingshot effects it got from passing Jupiter and Saturn.