Here is a quote from Susskind's the Cosmic Landscape: I'm having a very difficult time wrapping my head around this. I'm pessimistic that it can be explained how gluon's hold quarks together because in order to explain x you need to break x down into parts and as of yet we do not know of anything smaller than a gluon and a quark. How did they even discover this? Did they literally observe the gluons occupying space between quarks? Is it proven through equations? Further, is it simply an assumption that if bosons are occupying space between gluons, then they must be that which prevents the quarks from escaping from the nucleus, or is there more rigor involved? Also regarding the fact that bosons can occupy the same space. I'm having trouble understanding that. How can you observe two bosons in the same space? It seems true by definition that there is one thing per one unit of space. Did they discover this through equations or observations?