The fact is, this curtain of light isthere, but our eyes cannot see it. Due to the expansion of the universe, the wavelengths of these hot, early photons have been stretched over 1,100 times longer than their original wavelengths. The high-energy, luminous backdrop of the early universe, is today filled with relatively cool, microwave photons, invisible to the human eye after being stretched by the expanding fabric of our universe for over 13 billion years.