Сложившееся is past perticiple (neutral gender) from сложить. Сложить would mean put things in one place and/or in a particular order. That's the meaning better suited for your case anyway. For example: when you put a lot of things in a backpack while also trying to do that in a space efficent way that'd be сложить вещи. Same thing can be said about a puzzle, like you have a lot of pieces you need to put together in the right order to get a complete image.

So just think about сложившееся мнение is an opinion that was "put together" over time by adding pieces of evidence or facts to the whole picture. Usually it is used to denote an opinion that most people now hold, like in your case, but it is in no way restricted to just a "popular" opinion. You can have your own "сложившееся мнение" after all.

Also take a note:

You can also say мнение сложилось. Like a person observes something and draws an opinion from what they have seen so far:

У меня сложилось мнение, что ему нельзя доверять!

Another note: the imperfective form of this verb is складываться, not to be used in past tense.

У меня складывается мнение, что ему нельзя доверять!

Hope this helps, by the way people there gave you good answers, just made it a bit limited to your particular case.