There are gaps between the molecules of a substance and they do not stick to each other because …?

The strength of the interaction of molecules depends on the distance between them. At relatively large distances, this force is zero. With decreasing distance, the force of attraction between molecules begins to increase, due to which solids and liquids are formed. Valence electrons play an important role in the emergence of this force. The molecules do not finally stick together by repulsive forces, which grow rapidly with decreasing distance. This begins to manifest the effect of all the electrons of one molecule on all the electrons of another molecule. The repulsive force decreases rapidly with increasing distance between the molecules, because they are generally electrically neutral.
Answer: With a strong approach of molecules, repulsive forces prevail.