Big Bang

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Across
  1. 4. Fusion process of atomic particles being smashed together
  2. 8. a point of infinite mass and heat
  3. 9. the second simplest atom
  4. 10. the universe is doing this even now
  5. 11. the theory galaxies are moving away at constant speeds
  6. 13. how fast light can travel in the vacuum of space
  7. 15. the amount of matter in an object
  8. 18. created all complex elements through nuclear fusion
Down
  1. 1. clusters of billions of starts and other pieces of matter
  2. 2. the act that brought the universe into existence
  3. 3. all existing matter, space and energy as a whole
  4. 5. the universe started really hot but now it is ____
  5. 6. the densest things in the universe
  6. 7. smallest particles that everything in the world is made of
  7. 8. a system of planets orbiting a sun
  8. 9. the simplest atom
  9. 12. the universe is _______ of years old
  10. 14. force of attraction that keeps us stuck on Earth
  11. 16. the star at the center of our solar system
  12. 17. to move around another object in a curved path