"We still don't know for sure what it is, but at least one thing we thought we knew about the present day universe isn't true."
According to Quantum FFF Theory, each open star cluster -and former star cluster stars like the sun- has its own dual stellar black holes ( undiscovered by now) emitting high energetic particles, so it is not a surpize that they concluded:
"The fact that this accounting works in the early universe but falls apart locally has scientists puzzled
But the number of known quasars is far lower than needed to produce the required light" see:
https://www.researchgate.net/publication/377066296_6x9_2023-B-17-nieuw_Splitting-BH-BB-12
SEE: http://www.dailygalaxy.com/my_weblog/2016/07/enigma-of-light-in-our-universe-some-of-it-could-be-coming-from-a-currently-unknown-exotic-source-we.html