Not really that much of a surprise there would be exo-moons but then our being able to find them, well now, that is pretty amazing! This could be the case in the Kepler-1625 system; the Hubble Space Telescope may have revealed a moon orbiting one of the system’s planets.
This thing is, this “moon” is pretty big, about the size of Neptune with the planet being about three Jupiter-masses. Is it a moon or is this some sort of binary planet system? I am not saying this is not a moon mind you, more of a “definitions-thing” you see. It’s all very intriguing to me.
Space.com has a great story on this and addresses the question.