In 1054, a star exploded. Actually, it exploded about 6000 years previous, but the light from the explosion only reached Earth in 1054. The expanding cloud of gas from this explosion is now known as the Crab Nebula, in the constellation Taurus.
At the time the light from this supernova reached Earth, the Catholic Church was the ruling political power in Europe, and their dogma, regarding any understanding of nature, was that good Christians must always defer to Aristotle, known as "The Philosopher" at the time, for any question about what was true.
This supernova was quite bright: It was visible by day for most of a month, and bright enough to read by it, at night. It was brighter than the full moon for almost two months.
This event was recorded by:
... which is why we know the year it happened.
Since heresy was such a serious, life-threatening charge, when European Christendom saw what must have been an unavoidably noticeable light in the sky, they deferred to Aristotle, who had pronounced the Universe to be "eternal and unchanging."
Hence, this obvious change in the Universe was Somebody Else's Problem - everyone pretended like it wasn't there, rather than risk excommunication for heresy by talking or writing about it. It couldn't have been invisible - even clouds would not have masked its light. But everyone acted as though it was, and it's completely missing from European records: The Emperor's New Clothes, writ astronomically.
In 1731 this event finally made the history books in Europe, when the English amateur astronomer John Bevis first noticed it in his telescope. It later became the first celestial object listed in the catalog created by Charles Messier: M1