The moon, as is often common in natural satellites, is in synchronous (or synchronous) rotation around the earth. This means that it uses the same time in a complete translation around our planet that in a rotation around its own axis, and that is the reason why the face we see is always the same. In the case of the moon, each of these two movements, translation and rotation, has a period of something less than four weeks (27.32 days).
This synchronous rotation is the effect of what we call a tidal link between planet and satellite. When a satellite has movement around another body of greater mass, a readjustment occurs, both in the orbit and in the mass distribution, of both bodies. If one of them is much more massive than the other and also its orbit is approximately circular, as in the case of Earth and the moon, it is the smallest that suffers a greater readjustment to the movement of the other and that gives place to the synchrony. In the case of two bodies with similar masses or less distance between them, as occurs between Pluto and its expensive satellite, a reciprocal tidal coupling is quickly reached in which also the rotation lasts the same for both bodies.
The effect of the tidal coupling in the case of the moon has caused that its spherical shape (consequence of its own gravitation) is slightly flattened. This form is bulging in the directions of the imaginary axis between the moon and the Earth and deflects it, almost imperceptible to us, of the spherical shape. In addition to this elongation, there has also been a redistribution of materials that favors that one of the faces accumulate the densest material and remains oriented by gravitational attraction towards the most massive body, the Earth, around which orbits.
The coupling also has effects on our planet. Its rotation has been frenándose since the formation of the Earth-moon system, since initially a terrestrial day lasted about five hours. Moreover, as we know, the displacement of the oceans occurs in the two directions of this imaginary axis between the Earth and the moon and that gives rise to the oceanic tides. In a large body of water we easily appreciate the effect and something equivalent occurs in the solid part of both bodies.
The mating effect continues today. The moon moves away from the earth at a speed of about three centimeters a year and tends to a reciprocal tidal coupling, such as Pluto and Charon. When that happens, if it does happen in the very distant future, a geostationary orbit will be achieved in which the moon will only be visible from a hemisphere of the planet and a day on Earth will be as long as the time that the moon takes to take a full turn around it .
During the formation of the solar system there were numerous violent phenomena and even collisions. Thanks to the lunar material samples collected by the Apollo missions we know that the moon was formed as a result of one of these impacts: the collision of a planetoid of the approximate size of Mars with a land still in formation. The coup had such magnitude that it detached to part of the terrestrial mantle, fused the material of the outer layers of both bodies and generated an enormous amount of vaporized rock. This material formed a ring around the initial Earth that over time gave rise to the current Earth-moon system. That happened shortly after the formation of the solar system and the initial land, about 4 billion years ago.
While the Earth-Moon system was formed, the two faces were defined as both our planet and its satellite were getting cold and the materials were distributed on the moon. The Earth-oriented face contains the so-called Lunar seas (also known as Maria which is the plural of the Latin word mare). They are areas of basaltic lava, now solid, formed by denser materials. The hidden face, which we do not see from the ground, contains lighter materials and is covered with impact craters because it is the one that suffers the greatest exposure to the external bombardment.