Measurement of Structure in the Cosmic Microwave Background Radiation Has

Multiple Choice

Measurement of structure in the cosmic microwave background radiation has recently indicated that the universe is flat (on the cusp between a closed and an open universe) , but the measured density of detected matter and radiation is only 20-40% of the critical density required for a flat universe. In what form is the other 60-80% of the "matter" thought to be?

A) myriads of small primordial black holes whose gravitational effects are spread throughout the universe and which emit no radiation
B) dark energy emitting no visible radiation
C) antimatter, which generates a negative gravitational effect and emits radiation only if it meets matter and is annihilated
D) neutrinos, which have very little rest mass and are very difficult to detect, but are very abundant