[optimusprinceps]

What exactly is propelling Voyager through interstellar space?

[alphydan]

Nothing, it's just momentum (or newton's first law of motion, https://en.wikipedia.org/wiki/Newton's_laws_of_motion)

[chton]

The speed it already has. There's practically no friction in space, so any speed it picked up from its engines and gravitational slingshots is kept. Newton's first law.

[batbomb]

and 30 years of any radiation pressure.