Why don’t the speedy Voyager space probes bump into anything?

The long-running series in which readers answer other readers' questions on subjects ranging from trivial flights of fancy to profound scientific and philosophical concepts

How is it possible that the Voyager space probes, travelling at 15.4km a second or even faster, manage not to crash into anything on their journeys? Presumably even hitting the slightest space debris at that speed would obliterate them.

Jon Lloyd, London SE23