If you attempt to push the orbital electrons of two atoms into the same volume of space, most of the electrons will need to be promoted to much higher energy states. You might call it an “exclusion” force, because it comes from the exclusion principle —”One of the nice properties to come out of the depths of quantum physics is the “Pauli Exclusion Principle“. You could argue (successfully) that the Pauli exclusion principle is responsible for all of the complex chemistry of the universe. So, it ends up in a high energy state, because it can’t find any that are lower. When two atoms are brought very close together their electron orbitals start to overlap and the atoms start to “share” electron orbitals.