Because there are so many neutrinos in the universe, and just a little rest mass adds up quickly to become a substantial factor in the dynamics.
There are about as many neutrinos as there are cosmic background photons: about 400 per cubic centimeter. Even with a mass as little as 7 electron volts (an electron's rest mass in the same units is 511,000 electron volts) this multiplies out to a total mass that is 10 - 20 percent of the critical density of the universe.
Copyright 1997 Dr. Sten Odenwald
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