I have looked into this myself and I do believe there is a network at the top where evrything interlinked and politicians know business leaders, Journalist etc. But you cannot say for sure that this theory is true.
The Illuminati was a secret group within the Freemasons in Europe in the late 1700's. That's the original root of this. It was real at one point in history.
The thing is, there's nothing to say that the group exists today, and there's lots to say that it doesn't.
The biggest flaw of "secret societies" is that once their secret is out, they can't get it to disappear again. The Illuminati has been known about for some time, so their greatest shield and defense is gone.
The idea of this group is so big today because of various works of literary fiction that have been made in the last few decades. People have read those books and somehow thought they were real, or were based on reality, but they're not.
It didn't originate with the movie but the movie was a big influence on people. It helped spread the conspiracy.
This is also why some people (especially young impressionable people) believe in ghosts, vampires and alien abductions. It's because of books, movies and TV.
It's just fantasy and superstition, though.
That's all there is to it.
The Illuminati started the rumor themselves. Then, they started the attacks on the rumor
as being a conspiracy theory Then, they started the defenders of the theory responses.
Now, everyone is so confused, no one knows if it's true or not! Smart fellows- those
Illumnati........or are they?