Solve the crossword and give me the topic name

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Across
  1. 5. closer to the Sun, planets move ...
  2. 7. the topic of this crossword
  3. 9. a place where gravity is so strong that nothing may escape
  4. 11. rocks in space share no similarity in their shapes, this is described as ...
  5. 12. the name of our galaxy
Down
  1. 1. the Moon is called a natural ... of the Earth
  2. 2. rocks in space that circle around the Sun
  3. 3. when a small asteroid impacts the Earth, it causes a hole called ...
  4. 4. the space where there is no particles
  5. 6. a cloud of gas or dust in space
  6. 8. name given to those galaxies that have oval shapes
  7. 10. the force hold everything in space together