the added contribution of the dark matter that has an apparent gravitational effect in galaxy clusters (see Chapter 21, "The Milky Way: Much More Than a Candy Bar"), we can account for up to 30 percent of the critical density.
It is possible, though, that the other 70 percent of the density of the universe is not in mass, but in energy. Remember that energy and mass can be interchanged through the process described by E = mc2. Einstein's "blunder" (see Chapter 25, "What About the Big Bang?") may point the way. If the expansion of the universe is accelerating, and the universe has "critical density," these might both be explained by energy contained in the vacuum of space.
Why this obsession with critical density? Well, one argument goes that it is unlikely that the universe would have a density so close to being critical without actually being critical.
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