I suppose that the Universe we can observe and cognize consists of self-developing hierarchically co-subordinate and genetically relative systems of cosmic bodies. Each of the systems is distinguished by the qualitative state of substance composing its bodies but main evolution mechanisms are common to all the systems.
What is common to all the systems? The central most massive body of every system is a parental body for its younger members. The central massive body of every system at intervals conceives in its interiors new cosmic bodies to put them into initial (circum-parental) orbits. In other words, the central body of every system stimulates newborn bodies to move under their own momentum (along a straight line) and to rotate about its axis due to its own gravitational attraction force. But the central gravitating body constantly and intensely loses its mass (photon and corpuscular). So, the orbits of newborn bodies are (must be) spiral-like rather than closed due to the law of universal gravitation. It means that newborn bodies move away from their parental body that is every system expands.
It should be noted that every cosmic body is imponderable, i.e. it undergoes gravitational interaction with other bodies.
What is the cause? The cause is that imponderability is provided by a certain orbital rate of the body controlled by the law of universal gravitation. The rate depends on radius of the body orbit relative to the system's central body: the more is the orbit radius, the less orbital rate the body needs to be imponderable. It can be seen from the motion of the Solar System's planets, satellites of giant planets and from the Earth's artificial satellites launched by man. The motion of both artificial satellites and cosmic bodies is governed by the same laws.
I suppose that our Universe consists of hierarchically relative systems as follows:
The first system: the Origin - the central gravitating body of our Universe, and its born nuclei of super-galaxies (the Origin is not considered in the present work).
The second system: nuclei of super-galaxies and their born nuclei of galaxies.
The third system: nuclei of galaxies and their born stars.
The fourth system: stars and their born planets including giant planets, comets, meteorites.
The fifth system: giant planets and their born satellites, comets, meteorites.
One specific feature of the system hierarchy should be noted. The central parental body of every system mentioned has its own sphere of gravity response predetermined by its mass. Radius of the Earth's gravity response sphere is known to be 930 thous km. The Sun and the Earth show nearly equal gravity response to interplanetary stations at the boundary of this sphere. If a station comes out of the sphere of the radius mentioned, it is already subject to gravity response of the Sun. The Sun, with its own sphere of gravity response to planets is, in its turn, involved into the sphere of gravity response of the...