It does not need to be molecules - atoms and subatomic particles will do the same. A “starship” or even one traveling at very high speeds is like a target in a particle accelerator. The difference is the target - i.e., the spacecraft is moving at relativistic, near relativistic or even high speeds. The particle is essentially stationary. It’s estimated that deep space matter density is roughly 1 hydrogen atom (a proton!) per cubic centimeter (It’s likely higher!). The matter density would of course be higher in interstellar clouds. That hydrogen atom would be slamming into every square centimeter of the spacecraft’s hull. Then there’s electromagnetic radiation - gamma rays, etc. Everything with charge you could deflect with an electromagnetic field (Note that would draw from your spacecraft’s power source!) the photons, you have much more of a problem! Density of hull material only solution known now even that over time will be a problem, eventually it would absorb enough energy and radiate it back inside the craft. Huge huge engineering problems here!
So are you one who doesn’t believe we’ve been to the moon?