  There is now strong evidence that the main contribution to the cosmic energy
density is not due to matter, but to another component with negative pressure.
Its nature is still unknown: it could be the vacuum energy, manifesting itself
as a positive cosmological constant with $w \equiv P/\rho c^2 = -1$
($\Lambda$CDM model), a spatially inhomogeneous and dynamically evolving form
of energy with $-1 < w < 0$ (quintessence, QCDM model), or a dark energy
component with $w < -1$, such as a quantized free scalar field (VCDM model).
After presenting simple redshift--distance formulae, which are useful for
comparing observations with theoretical predictions without numerical
integration, I discuss the behaviour of different $w < 0$ models in the context
of the Alcock--Paczy\'nski geometric test and the statistics of gravitational
lensing.
