### Clue: Clouds in 21 unable to be dispersed, sucking in energy (7)
### Answer: NEBULAE 
 ### Explanation: Definition: Clouds in 21
  **Anagram of**(to be dispersed) UNABLE **containing**(sucking in) E(in physics, abbrev. for $$e$$nergy).
Answer:  Clouds of particles and gases in outer space(answer to 21across).