Dark matter is inferred because which statement is true?

• A. Most of the universe (~80%) consists of dark matter.
• B. Dark matter glows in visible light.
• C. It can be detected easily using simple instruments.
• D. It forms only in the early universe and disappears.

Answer: (A)

Dark matter is inferred because gravity reveals more mass than what we can see with our eyes. In many galaxies, stars and gas at the edges orbit just as fast as those near the center, which wouldn’t happen if all the mass were visible. That implies there’s extra mass spread around the galaxy that we can’t see with light. Galaxy clusters show even stronger evidence: motions of galaxies inside clusters and the way light from background galaxies is bent (gravitational lensing) require more mass than the visible galaxies and hot gas account for. The cosmic microwave background also points to a large unseen matter component that helped structure form in the early universe. Since dark matter doesn’t emit, absorb, or reflect light, we infer its presence from these gravitational effects, not direct detection. The upshot is that a substantial, unseen mass component curves the motions and light in the cosmos much more than visible matter alone can explain. The other statements aren’t supported by observations: dark matter isn’t known to glow in visible light, it’s not detectable with simple instruments, and it isn’t thought to form only early on and then disappear.