For decades , the determination of the mean density of matter ( \Omega _ { M } ) has been tied to the distribution of light .
This has led to a “ bias , ” perhaps as large as a factor of 2 , in determining a key cosmological parameter .
Recent measurements of the physical properties of clusters , cosmic microwave background ( CMB ) anisotropy and the power spectrum of mass inhomogeneity now allow a determination of \Omega _ { M } without “ visual bias. ” The early data lead to a consistent picture of the matter and baryon densities , with \Omega _ { B } = 0.039 \pm 0.0075 and \Omega _ { M } = 0.33 \pm 0.035 .