From here: https://in.answers.yahoo.com/question/index?qid=20100421234931AAnRi1H Please could someone tell me exactly what is causing the repulsion? When it says size of groups, what does it mean? What groups? What's a group? Shouldn't larger things attract, not repel? Is it a structural issue? Thanks for any help with a more simple explanation, I've checked Wiki and I'm not finding it much use tbh.