  We derive the bulk viscous time scale of neutron stars with quark matter
core, i.e. hybrid stars. The r-mode instability windows of the stars show the
theoretical result accords with the rapid rotation pulsar data. The fit gives a
strong indication for the existence of quark matter in the interior of neutron
stars. Hybrid stars instead of neutron or strange stars may result in
submillisecond pulsars if they exist.
