Measurement of structure in the cosmic microwave background radiation has recently indicated that we live in a flat universe between a closed and an open universe, and yet the measured density of detected matter and radiation is only 20% to 40% of the critical density required for a flat universe. In what form is the other 60% to 80% of the "matter" likely to be?
A)antimatter, which generates a negative gravitational effect and emits radiation only if it meets matter and is annihilated
B)neutrinos, which have very little rest mass and are very difficult to detect, but are very abundant
C)large numbers of small primordial black holes, whose gravitational effects are spread throughout the universe and that emit no radiation
D)dark energy, emitting no radiation and generating no detectable gravitational effects

Question

Quizplus · Accepted Answer

The missing mass is likely in the form of dark energy, which is believed to be responsible for the observed accelerating expansion of the universe. Dark energy does not emit radiation and its gravitational effects are difficult to detect, which is why it has not been directly observed.

Measurement of Structure in the Cosmic Microwave Background Radiation Has