Not sure why that would be. Given that there are stars exploding, and universes colliding, would that
not mean there must be debris in between the planets? Is there something that I am missing?
Picture all stars, planets, asteroids and other matters in the
universe evaporate into atoms which then diffuse into the
space evenly. The universe will remain pretty much a perfect
vacuum. It will be something like one hydrogen atom in one
cubic meter. Density of the best laboratory vacuum is easily larger
than that by more than one million-fold.
Since the universe is pretty much a void the probability of
hitting on an asteroid in the space between stars and between
galaxies even when travelling near light speed is essentially nil.
As the spacecraft is approaching the destination star system
it will have to decelarate to a minute fraction of light speed.
Asteroids do not shot randomly in every direction in space
but orbit around planets. Hitting on an asteroid has never been a
concern for space probes like Voyager 1 and 2. It will be the last
thing to worry about in the future manned mission to Mars.
I am fascinated by these topics, so any info will help.