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 20-40% 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 that 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

Question

Quizplus · Accepted Answer

The missing 60-80% of matter in the universe is thought to be dark matter and dark energy. Dark matter does not emit or absorb light, so it cannot be detected directly, but its presence is inferred from its gravitational effects on visible matter. Dark energy is a hypothetical form of energy that is believed to be responsible for the observed accelerating expansion of the universe. Both dark matter and dark energy are not visible and therefore would not contribute to the detected matter and radiation density. Option A is incorrect because the existence of primordial black holes is still uncertain, and even if they exist, they are not thought to constitute a significant portion of dark matter. Option C is incorrect because while antimatter does exist, it makes up a negligible fraction of the observed matter in the universe. Option D is incorrect because although neutrinos do have mass, they are still not enough to account for the missing matter in the universe.

Measurement of Structure in the Cosmic Microwave Background Radiation Has