  A Friedmann-Robertson-Walker cosmological model dominated by tachyonic --
faster-than-light -- dark matter can exhibit features similar to those of a
standard dark energy/dark matter or $Lambda$CDM model. It can undergo expansion
which decelerates to a minimum rate, passes through a "cosmic jerk," then
accelerates. But some features of a tachyon-dominated model are sufficiently
distinct from those of the standard model that the two possibilities might be
distinguished observationally. As a demonstration of concept, the
distance-redshift relation of such a model is compared here with some
observations of Type Ia supernovae. Other measures of the third time derivative
of the cosmic scale factor -- the true cosmic jerk -- might be found to test a
tachyonic-dark-matter hypothesis.
