Very, very small. But still substantial compared to interstellar space.
"Viscosity" is a measure of stresses created by the movement of particles against each other or other particles. Where there are very few particles of matter the viscosity is necessarily also very small, as there are much fewer particle interactions than in more dense areas.
Its SI unit is the Pa-s, or unit pressure times time. The key part here is that when the pressure is extremely small, there is very little viscosity.