  Recent attempts to explain the dark matter and energy content of the universe
have involved some radical extensions of standard physics, including
quintessence, phantom energy, additional space dimensions and variations in the
speed of light. In this paper I consider the possibility that some dark matter
might be in the form of tachyons. I show that, subject to some reasonable
assumptions, a tachyonic cosmological fluid would produce distinctive effects,
such as a surge in quantum vacuum energy and particle creation, and a change in
the conventional temperature-time relation for the normal cosmological
material. Possible observational consequences are discussed.
