Reionization as illustrated by data from the Hubble and Chandra space telescopes. Credit: NASA/CXC/Mark Weiss
An article recently published on Physorg tries to explain why the universe can be transparent, but it doesn't do a very good job in my opinion. Here's my reply on Facebook to someone who had trouble understanding what the article was trying to say:
Think of a window. Light can pass through it the same way that it passes through dark matter. Dark matter is that window, only it's everywhere and not just a 2D boundary. In the Dark Ages, shortly after the Big Bang, there wasn't enough light to pass through. The universe was completely dark until reionization made enough quantities of light to penetrate it.
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