  By studying the bispectrum of the galaxy distribution in the 2dF galaxy
redhsift survey (2dFGRS), we have shown that 2dFGRS galaxies are unbiased
tracers of the mass distribution. This allows us the break the degeneracy
intrinsic to power spectrum studies, between the matter density parameter
Omega_m and the bias parameter b, and to obtain an accurate measurement of
Omega_m: Omega_{m,z_{eff}}=0.27 \pm 0.06, a measurement obtained from the
2dFGRS alone, independently from other data sets. This result has to be
interpreted as Omega_m at the effective redshift of the survey z_{eff}=0.17.
Extrapolated at z=0 we obtain Omega_m=0.2 \pm 0.06 for the LCDM model. This
constraint on the matter density parameter when combined with cosmic microwave
background constraints on the flatness of the Universe, show firm evidence that
the Universe is dominated by a vacuum energy component.
