"What was actually heard was something totally unexpected. Permeating the Universe was a strong radio hiss, about six times more powerful than anyone expected. No-one knew what it was or what was causing it. This was eventually christened "Space Roar" and remains one of the "Great Unsolved Problems in Physics"."
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On my random walk through the interwebs I came across this article, which is low quality overall, but the space roar seems to be a real issue in radio astronomy - at least Wikipedia has an entry about it 8-).
I did not know about this "great unsolved problem in physics" until now, so please leave a comment if you happen to know more about it ...