CDM models with a smooth component

The inflationary prediction of a flat Universe is at odds with current determinations of the matter density $({\Omega }_M\simeq 0.2-0.4).$ This dilemma can be resolved if a smooth component contributes the remaining energy density $({\Omega }_X=1\mathrm{}-{\Omega }_M).\mathrm{}$ We parametrize the smooth component by its equation of state, $p_X=w{\rho }_X,$ and show that $X$CDM with $w\simeq -0.6,$ ${\Omega }_M\simeq 0.3,$ and $h\simeq 0.7$ is the best fit to all present cosmological data. Together, the position of the peak in the CMB angular power spectrum and the type Ia supernova magnitude-redshift diagram provide a crucial test of $X$CDM.