 We've mentioned dark matter several times in reference to the CMB. It was the largest matter component of the opaque universe at the time of decoupling. And it is the dark matter density anisotropies that produced the filament structures we see in the universe today. But this only works if the dark matter doesn't fly off at near the speed of light like neutrinos do. In this respect, it is said to be cold, cold dark matter, CDM for short. Using galaxy rotation rates, galaxy cluster dynamics, and gravitational lensing, the best estimates for the dark matter density parameter in the universe is 0.262. The baryon matter density parameter is measured to be around 0.048. Radiation, mostly from the CMB, is significantly less than that. The problem is that all of this isn't enough to account for our flat universe observations. But there is one component of the universe that we have not yet taken into consideration. And that is that empty space, the vacuum itself, has energy.