  We analyze dark pulse Kerr frequency combs in optical resonators with normal
group-velocity dispersion using the Lugiato-Lefever model. We show that in the
time domain these correspond to interlocked switching waves between the upper
and lower homogeneous states, and explain how this fact accounts for many of
their experimentally observed properties. Modulational instability does not
play any role in their existence. Furthermore, we provide a detailed map
indicating where stable dark pulse Kerr combs can be found in parameter space,
and how they are destabilized for increasing values of frequency detuning.
