that's just my opinion 

1. A phrase used after stating an irrefutable or widely accepted fact, often to deride the intelligence of the listener and to add a quality of snarkiness to the statement.
2. (that's just your opinion) A reply to a statement of irrefutable fact, used to place the statement-maker in the awkward position of having to back up their statement with further irrefutable facts. Often followed up with one's own ridiculous opinion.

Actually, two plus two *does* equal four, but that's just my opinion.

Sir Isaac Newton proved that gravity is the force that keeps us all on Earth.
Well, that's just your opinion. Personally, I think we're all just still here so we can read Homestuck.

that's just my opinion by astadelic December 6, 2011

but that's just my opinion 

"I know it's bigoted, ill-informed and offensive, but I'm going to say it anyway."

"Immigrants are bleeding the country dry, but that's just my opinion."

but that's just my opinion by Mentalfloss October 28, 2013