Why was the universe not transparent in its first 300,000 years? What would a clear box look like if it were filled

Question: Why was the universe not transparent in its first 300,000 years? What would a clear box look like if it were filled with just electrons and nothing else?
- Keywords:
  universe
Asked by eigenvector to Arttu, Ceri, James_M, Monica, Philip on 20 Jun 2011.
- James M Monk answered on 20 Jun 2011:
  
  It was too hot for neutral atoms to form – there was only plasma, or when it was even hotter there would not even have been protons and neutrons. In any case, there would have been free charged particles.
  
  Photons of light interact with charged particles, so they cannot pass very far through the plasma before being scattered. Thus any photons in the Universe from that time cannot reach us because they cannot pass through the plasma.

Question: Why was the universe not transparent in its first 300,000 years? What would a clear box look like if it were filled with just electrons and nothing else?