If the density of the universe equals the critical density, then the Universe must be ___________ ...

If the density of the universe equals the critical density, then the Universe must be ___________ (i.e., having zero curvature).
 
  Fill in the blank(s) with correct word

Question 2

Antiprotons and positrons are examples of ______________, which are not ordinary matter.
 
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Question 3

After recombination occurred, the Universe was plunged into a period of time known as the ___________, which lasted hundreds of millions of years until the first stars were born.
 
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Question 4

If in every direction we point to a star, then the conundrum that the sky is dark is known as ___________.
 
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Question 5

The initial high-temperature, high-density state from which time starts is known as the ___________.
 
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