RAS question
Dark matter is called 'dark' because it:
Correct answer: (B) Does not interact with electromagnetic radiation.
Dark matter is called dark because it does not interact with electromagnetic radiation by emitting, absorbing, or reflecting light.
Explanation
Dark matter is not dark because it appears black, exists only at night, or sits inside black holes. Detectability is the key issue. Dark matter does not emit, absorb, or reflect electromagnetic radiation, which makes it invisible, and it is detected only through gravitational effects. NASA Science says dark matter is called dark because it is invisible to us and does not absorb, reflect, or emit light. Dark matter is dark because it does not interact with electromagnetic radiation. About 27% of the universe is dark matter, about 68% is dark energy, and only about 5% is ordinary visible matter.
Why the other options are wrong
- (A) Dark matter is not a night-time phenomenon; it remains invisible because it does not emit, absorb, or reflect light.
- (C) Black holes are separate objects, while dark matter itself is termed dark because it is inferred through gravity and does not emit, absorb, or reflect light.
- (D) Dark matter is not black in colour; NASA says the name refers to invisibility, not to a visible colour.
Concept
Cosmology separates electromagnetic observation from gravitational inference. Unseen matter can still have physical effects through gravity.