It is well-known that two bodies attract one another by gravitation. The theory of Newton gives a formula for this attraction depending on the masses of the bodies. General relativity of Einstein explains gravitation as geometry. There exists no satisfactory explanation for the meaning of gravitation. A theory of gravitation in flat space-time gives non-singular, cosmological models. In the beginning of the universe, there is no matter and all the energy is uniformly distributed gravitational energy. This energy is attractive and converts to dark matter generating our universe. Gravitational energy is the reason for attraction and not the mass of the bodies. It is well known that each body is surrounded by a gravitational field. This field gives the attraction of bodies.