Eventually, he realized that it seemed to describe a new type of particle: one that’s neither a matter particle nor a force-carrying particle. There are the particles that make up matter, called fermions, and the particles that impart the fundamental forces, called bosons. The defining characteristic of fermions is that if you switch the positions of two fermions, their quantum state gains a minus sign. The presence of that measly minus sign has enormous ramifications. This feature prevents matter from collapsing in on itself—it’s why the electrons in every atom exist in “shells.” Without this minus sign, we couldn’t exist.