We infer dark matter's existence because it explains the observed motions of planets and galaxies even though it emits no light. This statement is:

• A. False
• B. True
• C. Cannot be determined from the information provided
• D. Irrelevant to cosmology

Answer: (B)

Unseen mass is inferred from how gravity shapes motion. Dark matter doesn’t emit light, so we can’t see it directly, but its gravity affects how fast stars orbit in galaxies and how galaxies move within clusters. The observed speeds and the way light is bent by gravity (gravitational lensing) require more mass than what we can see with telescopes. That extra, invisible mass is what we call dark matter, and it explains why these motions look the way they do. So the statement is true: we infer dark matter’s existence precisely because it accounts for observed motions while emitting no light. On planetary scales, dark matter’s effect is negligible, but the larger-scale dynamics are the strong evidence.