  Usually considered highly speculative, tachyons can be treated via
straightforward Einsteinian dynamics. Kinetic theory and thermodynamics for a
gas of ``dark'' tachyons are readily constructed. Such a gas exhibits density
and pressure which, for the dominant constituent of a suitable
Friedmann-Robertson-Walker spacetime, can drive cosmic evolution with features
both similar to and distinct from those of a standard dark-energy/dark-matter
model. Hence, tachyons might bear further consideration as a cosmic dark-matter
candidate.
