What Gravity Still Hides: teacher key

Pushes and Pulls, grade 12 edition, written for 12th grade and up. Friction is still not completely understood, and Einstein's relativity replaced Newton's gravity. NASA estimates that dark matter is about 27% of the universe as of November 2026, and what it is remains open.

Read the book. Print this page for the key.

Questions and answers

1. As of November 2026, about what share of the universe does NASA say is dark matter?
   • 5%
   • 68%
   • 27% (answer)
2. What did Fritz Zwicky see in the Coma Cluster in 1933?
   • Galaxies moved too quickly for the gravity of the matter we can see (answer)
   • Galaxies were not moving at all
   • Galaxies were colliding
3. What did Vera Rubin see at the outer edges of spiral galaxies?
   • Stars moving too slowly
   • Stars moving as if held by more gravity than visible matter supplies (answer)
   • Stars that gave off no light
4. Why is no constant-velocity frame of reference special, according to the passage on the first law?
   • Nothing at rest exists in the universe
   • Frames that are not accelerating are rare
   • Something at rest for one observer moves in a straight line for another (answer)
5. In the passage on dark matter, what are axions?
   • Proposed candidates for dark matter (answer)
   • Stars at the edge of galaxies
   • Gravitational waves
6. As of November 2026, what does NASA say about searches for dark matter candidates?
   • One candidate has been confirmed
   • Searchers have not yet found evidence that confirms any one (answer)
   • All candidates have been ruled out

Words

• observer: one who measures
• relativity: Einstein's gravity theory
• galaxies: star systems
• axions: proposed particles