According to the big bang theory, after the "bang," the universe remained dark until about later, when neutral atoms began to form.

300,000 years later when neutral atoms began to form

By drawing a diagonal in a square you are dividing the square into two congruent isosceles right triangles. Using Pythagorean Theorem we can find the length of the diagonal.

a squared + b squared = c squared
a and b represent the sides of the triangle (aka the length of a side of the square), and c represents the hypotenuse (aka the diagonal).
9 squared + 9 squared = c squared
81 + 81 = c squared
162 = c squared
take the square root of both sides
81= x