Rather unlikely. They would require enormous amounts of raw materials, would need to be constructed out of things that are so far beyond the state of the art th

Rather unlikely. They would require enormous amounts of raw materials, would need to be constructed out of things that are so far beyond the state of the art they might as well be magic, and would be dynamically unstable besides. Much smaller ring-shaped stations such as Banks' "Orbitals", quite likely, up to the limits of material science. But I'd be surprised if any true ringworlds existed in the entire Universe, or ever will.