Other forms: elephants in the room
Like a giant animal in a space far too small for it, the elephant in the room is a problem that everyone knows exists, but nobody wants to do anything about.
This phrase sometimes appears as elephant in the living room. If there was an elephant in your living room, it would be awfully hard to ignore it, but it would also be difficult to force it to leave. A metaphorical elephant in the room is something that's impossible to fully ignore, but also very uncomfortable or inconvenient to confront. So you just keep squeezing past it, tip-toeing around it, and trying to live with it.