In the Solar System, there are around 173 moons. It is not an exact number because with some there is no consensus on whether or not they are moons. Planets like Jupiter or Saturn have more than 50 moons. All the planets in the Solar System except Mercury and Venus (being too close to the Sun) have one. So the mystery of the exomoons baffles astronomers.
An exomoon is a moon outside the Solar System, in the same way, that an exoplanet is a planet outside the Solar System. That is to say, it orbits around a star that is not the Sun. We know hundreds of exoplanets, but no exomoons. How is it possible? If there are so many moons in the Solar System, the logical thing is that some exoplanets also have exomoons. But at the moment we have not discovered them.
In 2018, NASA announced that the Hubble Telescope had discovered a moon. 8,000 light-years from Earth, in the Swan Constellation, there is an exoplanet called Kepler-1625b. Around it orbits a supposed exomoon 4 times larger than Earth. But astronomers have little data and cannot confirm whether or not it is a former moon.
Whether or not it remains an exceptionally low number, in proportion to the number of moons in the Solar System. Why do we know so few exomoons? The most obvious answer is simply because we have not yet discovered them. Our observation instruments are not powerful enough to see them.
Keep in mind that the first confirmed exoplanet was discovered in 1992 (although indications were known to have existed long before), and in the past 30 years we have already discovered several hundred. We not only have increasingly powerful telescopes but new detection methods. For example, many of them have been discovered because they partially hide the light from the orbiting star when they pass in front of the telescope. It is a type of measurement that a few years ago could not be done.
For this reason, the discovery of exomoons is only a matter of time. The discovery of solitary moons, which have left the orbit of a planet, is not even ruled out. It is what is known as planets, or planets.
There are still many mysteries of the universe that resist us. But the exomoons will be one of the first that we solve.
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