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.