This is an English metaphorical idiom for an obvious truth that is either being ignored or going unaddressed. The idiomatic expression also applies to an obvious problem or risk no
one wants to discuss.
It is based on the idea that an elephant
in a room would be impossible to overlook; thus, people in the room who
pretend the elephant is not there have chosen to avoid dealing with the
looming big issue.
Prof G has used it in one context in the post below, it was presented to me in a few contexts at the weekend, I saw a Neuro use it to talk about Grey Matter lesions and the CCSVIers of course used it to lambast us.