be common knowledge

To be something that is generally or widely known, whether or not it has been verified or officially recognized. The fact that a healthy diet and regular exercise are the best defense against disease is common knowledge at this point. It's common knowledge that corporate interests play a major role in directing politicians and the laws they create. Sorry I didn't tell you about my promotion, I just figured it was common knowledge at this point.

be common/public ˈknowledge

be something that everyone knows, especially in a particular community or group: What do you mean you’re surprised he’s leaving? I thought it was common knowledge that he was looking for another job!