Vacuum is said to have an intrinsic impedance. What is the source of this? Impedance is quite understandable in bulk matter, but I'm unable to comprehend it's presence in vacuum.
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Impedance is a response of material to excitation. E.g., if we define an impedance of a conductor as a ratio of the applied bias to the resulting current, $$Z(\omega) =\frac{V(\omega)}{I(\omega)},$$ then nothing prevents us from defining it in the same way for vacuum. Naturally one would expect this impedance to be something simple like 1 (electromagnetic field passes without attenuation) or 0 (no electric current).
Specifically, one talking about the impedance of free space one often talks about the propagation of electromagnetic field $$Z = \frac{E}{H} = \sqrt{\frac{\mu_0}{\epsilon_0}},$$ which has dimensionality of resistance. The fact that in SI system of units this ratio is not $1$ is one of the main arguments against using this system of units in physics.
Roger V.
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