What is the largest known moon ever discovered by astronomers

What is the largest known moon in the universe ever discovered by human beings?

The only moons that we have discovered are in our solar system, and the largest of them is Ganymede, one of the four Gallilean moons that orbits Jupiter. The second-largest moon is Titan, which has a thick atmosphere and orbits Saturn.

Earlier on, it was thought that Titan was larger than Ganymede, but it was later discovered that this was only due to it's atmosphere.

Ganymede in our Solar System.

Technically, anything too large or massive around a planet cannot be called a satellite but instead we should call that a binary planet system since the center of mass will be well outside that of the larger planet's surface. We can say that the largest size a satellite/moon can achieve is close to about a hundredth of the largest possible planet mass which is 13 Jupiter masses. That's about 40 Earth masses. Even more massive than Neptune and Uranus combined. This scenario is very unlikely by the way since we don't even know what to call objects in that borderline mass.

I will not make this any longer since you snub detailed answers anyway.

Clear skies!

So far every moon we know of is in our solar system. We just don't have the technology to detect moons around planets that orbit other stars. The largest known moon is Ganymede, a moon of Jupiter, and is larger than the planet Mercury. Following a close second is Saturn's moon Titan.

Ganymede [GAN-ee-meed] is the largest moon of Jupiter and is the largest in our solar system with a diameter of 5,262 km (3,280 miles).

Ganymede, on of the Galilean moons of Jupiter, with a diameter ~5300km.

Rosie O'Donnell... wearing a short skirt.... dropped a candy bar in front of me...

I still have nightmares.