Our latest research shows that electromagnetic field will be generated when two objects collide with each other under an impact force, even if a tiny force. Here, objects were blown by wind to verify that there will be electromagnetic field generated by friction between the surface of the planet and the interstellar gas and dust (interstellar atmosphere). Ten objects that are easy to be found on the earth were chosen to prove it. The results show electromagnetic field will be generated when the wind blows these objects. The stronger the wind blows, the stronger the electromagnetic field will be. Based on this new discovery, we speculate that electromagnetic field can be generated by the friction between the surface of galaxy and the interstellar atmosphere. The ultra-high-speed rotation and revolution of galaxies leads to super strong electromagnetic field and other energy field, which resulted in the super strong gravity. The generated energy radiates outward in the form of electromagnetic waves or other waves that can’t be detected now, but is distributed throughout the outer space. That is what scientists called dark energy and dark matter. The resulting gravity is called external gravity here. The centripetal force is the result of external gravity and Newtonian gravitation according to the existed outward gravity. On this basis, we perfect and supplement the Newton's gravity. Newton's formula of universal gravitation is based on ideal conditions that objects of the system are not affected by external forces. Neglecting the interaction of other planets, we give two simple models to discuss the relationship between the centripetal force, the universal gravitation and the external gravitation produced by the interaction between sun and a planet. The existence of external and inherent gravity explains the existence of dark matter, and the fact that the rotation speed of galaxies is different from that calculated by universal gravitation. It is a good interpretation of some discovered phenomena about dark matter in the past. Where there are moving galaxies, there are dark matter and dark energy. Dark matter and dark energy are everywhere and flooded in the outer space. The centripetal force of planetary motion is the result of the combined action of universal gravitation and electromagnetic force.