Every atom in the Universe contains at least one electron. The simplest atom, Hydrogen, contains one proton and one electron. Electrons are special because they are the lightest of their kind (fermions) which means they are stable: they do not decay into other particles. They have a negative electric charge and a small mass of 0.511 MeV (Mega electronVolts) which is 0.0000000000000000000000000000009 grams.