  We show that the number of observed voids in galaxy redshift surveys is a
sensitive function of the equation of state of dark energy. Using the Fisher
matrix formalism we find the error ellipses in the $w_0-w_a$ plane when the
equation of state of dark energy is assumed to be of the form $w_{CPL}(z)=w_0
+w_a z/(1+z)$. We forecast the number of voids to be observed with the ESA
Euclid satellite and the NASA WFIRST mission, taking into account updated
details of the surveys to reach accurate estimates of their power. The
theoretical model for the forecast of the number of voids is based on matches
between abundances in simulations and the analytical prediction. To take into
account the uncertainties within the model, we marginalize over its free
parameters when calculating the Fisher matrices. The addition of the void
abundance constraints to the data from Planck, HST and supernova survey data
noticeably tighten the $w_0-w_a$ parameter space. We thus quantify the
improvement in the constraints due to the use of voids and demonstrate that the
void abundance is a sensitive new probe for the dark energy equation of state.
