Was it hiding behind dark matter? /s
Interestingly, it may, to some degree, diminish the need for dark matter, at least in terms of the grand scale, if there turns out to be many more galaxy clusters than they previously thought.
From the article:
"McDonald and his colleagues believe the discovery of this hidden cluster shows there may be other similar galaxy clusters hiding behind extremely bright objects that astronomers have miscatalogued as single light sources.
The researchers are now looking for more hidden galaxy clusters, which could be important clues to estimating how much matter there is in the universe and how fast the universe is expanding."
We all know or should know that Dark Matter matters.